AMERICAN TOWER CORP /MA/
S-1/A, 1999-02-08
COMMUNICATIONS SERVICES, NEC
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<PAGE>
 
    As filed with the Securities and Exchange Commission on February 8, 1999
                                                      Registration No. 333-71939
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
 
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
 
                                ----------------
 
                                AMENDMENT NO. 1
                                       To
                                    FORM S-1
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
 
                                ----------------
                           AMERICAN TOWER CORPORATION
             (Exact name of registrant as specified in its charter)
 
         Delaware                     4899                   65-0723837
     (State or other      (Primary Standard Industrial    (I.R.S. Employer
       jurisdiction       Classification Code Number)   Identification No.)
   of incorporation or
      organization)
 
                             116 Huntington Avenue
                          Boston, Massachusetts 02116
  (Address, including zip code, and telephone number, including area code, of
                   registrant's principal executive offices)
 
                                ----------------
 
                                STEVEN B. DODGE
                           American Tower Corporation
                             116 Huntington Avenue
                          Boston, Massachusetts 02116
                                 (617) 375-7500
 (Name, address, including zip code, and telephone number, including area code,
                             of agent for service)
 
                                ----------------
<TABLE>
<S>                          <C>
         Copy to:                                Copy to:
  NORMAN A. BIKALES, ESQ.                JOHN T. BOSTELMAN, ESQ.
 Sullivan & Worcester LLP                  Sullivan & Cromwell
  One Post Office Square                     125 Broad Street
Boston, Massachusetts 02109              New York, New York 10004
</TABLE>
 
                                ----------------
 
  Approximate date of commencement of proposed sale to the public: As soon as
practicable after the effective date of this Registration Statement.
 
  If any of the securities being registered on this form are to be offered on a
delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, check the following box. [X]
 
  If this form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, check the following box and
list the Securities Act registration statement number of the earlier effective
registration statement for the same offering. [_]
 
  If this form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [_]
 
  If this form is a post-effective amendment filed pursuant to Rule 462(d)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [_]
 
  If delivery of the prospectus is expected to be made pursuant to Rule 434,
check the following box. [_]
 
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>
 
                           AMERICAN TOWER CORPORATION
                       REGISTRATION STATEMENT ON FORM S-1
 
                                    PART II
 
                   INFORMATION NOT REQUIRED IN THE PROSPECTUS
 
Item 13. Other Expenses of Issuance and Distribution.
 
<TABLE>
       <S>                                                             <C>
       Securities and Exchange Commission fee......................... $  3,549
       New York Stock Exchange annual fee.............................    5,000
       New York Stock Exchange listing fee............................    5,000
       NASD Review fee................................................    1,776
       Printing and engraving fees....................................   10,000
       Accountants' fees and expenses.................................   50,000
       Legal fees and expenses........................................   50,000
       Miscellaneous..................................................   10,675
                                                                       --------
       Total.......................................................... $136,000
                                                                       ========
</TABLE>
  The expenses shall be borne by the Selling Stockholder.
 
Item 14. Indemnification of Directors and Officers.
 
  Section 145 of the DGCL provides, in effect, that any person made a party to
any action by reason of the fact that he is or was a director, officer,
employee or agent of ATC may and, in certain cases, must be indemnified by ATC
against, in the case of a non-derivative action, judgments, fines, amounts paid
in settlement and reasonable expenses (including attorney's fees), if in either
type of action he acted in good faith and in a manner he reasonably believed to
be in or not opposed to the best interests of ATC and, in a non-derivative
action, which involves a criminal proceeding, in which such person had no
reasonable cause to believe his conduct was unlawful. This indemnification does
not apply, in a derivative action, to matters as to which it is adjudged that
the director, officer, employee or agent is liable to ATC, unless upon court
order it is determined that, despite such adjudication of liability, but in
view of all the circumstances of the case, he is fairly and reasonably entitled
to indemnity for expenses.
 
  Article XII of ATC's By-Laws provides that ATC shall indemnify each person
who is or was an officer or director of ATC to the fullest extent permitted by
Section 145 of the DGCL.
 
  Article Sixth of ATC's Restated Certificate states than no director of ATC
shall be personally liable to ATC or its stockholders for monetary damages for
breach of fiduciary duty as a director, except for (i) breach of the director's
duty of loyalty to ATC or its stockholders, (ii) acts or omissions not in good
faith or which involve intentional misconduct or knowing violation of law,
(iii) liability under Section 174 of the DGCL relating to certain unlawful
dividends and stock repurchases, or (iv) any transaction from which the
director derived an improper personal benefit.
 
Item 15. Recent Sales of Unregistered Securities.
 
  Pursuant to that certain American Tower Systems Corporation Stock Purchase
Agreement, dated as of January 8, 1998, by and among ATC and the Purchasers
thereto, ATC consummated an equity financing involving the issuance of an
aggregate of 1,350,050 shares of Class A Common Stock, 4,649,950 shares of
Class B Common Stock and 2,000,000 shares of Class C Common Stock, each at
$10.00 per share, in exchange for $30.6 million in cash and $49.4 million of
notes secured by common stock of American Radio Systems Corporation having a
market value of not less than 175% of the principal amount and accrued and
unpaid interest on such notes.
 
  Pursuant to that certain Agreement and Plan of Merger, dated as of November
21, 1997, as amended, by and among ATC, American Tower Systems, Inc., a
Delaware corporation (now known as American Towers,
 
                                      II-1
<PAGE>
 
Inc., "ATI"), Gearon & Co., Inc., a Georgia corporation ("Gearon") and J.
Michael Gearon, Jr., pursuant to which Gearon was merged with and into ATI,
with ATI as the surviving corporation, in January 1998 ATC issued an aggregate
of 5,333,333 shares of Class A Common Stock and paid approximately $32.0
million in cash or assumed liabilities for an aggregate agreed upon
consideration of approximately $80.0 million.
 
  On June 4, 1998, ATC issued in a private placement to institutional investors
300,000 shares of Series A Pay-In-Kind Preferred Stock, $1,000 liquidation
preference per share, ("Interim Preferred Stock") pursuant to the Interim
Financing Agreement.
 
  Pursuant to the certain Agreement and Plan of Merger, dated as of April 14,
1998, by and among ATC, ATI, Intracoastal Broadcasting, Inc. ("Intracoastal")
and the stockholders of Intracoastal, pursuant to which Intracoastal was merged
with and into ATI, with ATI as the surviving corporation, in June 1998, ATC
issued an aggregate of 720,000 shares of Class A Common Stock and issued its
non-recourse note in the principal amount of approximately $12.0 million in
exchange for a broadcasting tower and notes in an aggregate principal amount
equal to the principal amount of the ATC note.
 
  Pursuant to the certain agreement and Plan of Merger, dated as of October 16,
1998, by and among ATC, ATI and Wauka Communications, Inc. ("Wauka"), pursuant
to which Wauka was merged with and into ATI, with ATI as the surviving
corporation, in October 1998, ATC issued an aggregate of 1,430,881 shares of
Class A Common Stock as part of the consideration in the merger.
 
  On February 4, 1999, ATC privately sold to Credit Suisse First Boston
Corporation 500,000 shares of Class A Common Stock at $26.31 per share, the
closing price of the Class A Common Stock on the NYSE on such date. In
connection with such sale, Credit Suisse First Boston Corporation was granted
certain registration rights.
 
  All of the shares referred to in the foregoing paragraphs were issued by ATC
in reliance on the exemption from registration provided by Section 4(2) of the
Securities Act of 1933, as amended (the "Securities Act"). Each holder
represented that it was acquiring its shares for investment purposes and not
with a view to distribution within the meaning of the Securities Act. The stock
certificates issued to all such holders bore restrictive legends. No commission
or other remuneration will be paid or given by ATC directly or indirectly in
connection with any of the foregoing transactions.
 
Item 16. Exhibits and Financial Statement Schedules.
 
  Listed below are the exhibits which are filed as part of this registration
statement (according to the number assigned to them in Item 601 of Regulation
S-K). Each exhibit marked by a (*) is incorporated by reference to ATC's
Registration Statement on Form S-4 (File No. 333-46025) filed on February 10,
1998. Each exhibit marked by a (+) is incorporated by reference to ATC's
Registration Statement on Form S-1 (File No. 333-50111) filed on May 8, 1998.
Each exhibit marked by a (++) is incorporated by reference to the filing of
Amendment No. 2 to ATC's Registration Statement on Form S-1 (No. 333-52481)
filed on June 30, 1998. Each exhibit marked by a (**) is incorporated by
reference to the filing of ATC's Current Report on Form 8-K filed on November
30, 1998. Each exhibit marked by a (***) is incorporated by reference to the
filing of ATC's Current Report on Form 8-K filed on January 8, 1999. Exhibit
numbers in parenthesis refer to the exhibit number in the applicable filing.
Each exhibit marked by a (+++) is incorporated by reference to ATC's
Registration Statement on Form S-4 (File No. 333-76083) filed on January 15,
1999. Each Exhibit marked by a (++++) is incorporated by reference from
Registration Statement (333-70881) on January 21, 1999. Each exhibit marked by
a (****) is incorporated by reference to the filing of Amendment No. 1 to ATC's
Registration Statement on Form S-1 (File No. 333-70881) filed on February 3,
1999. Each Exhibit marked by a (+) is incorporated by reference from the
initial filing of this Registration Statement (No. 333-71939) on February 5,
1999.
 
 
                                      II-2
<PAGE>
 
<TABLE>
<CAPTION>
 Exhibit                                                        Exhibit  File
   No.                  Description of Document                      No.
 -------                -----------------------                 --------------
 <C>      <S>                                                   <C>
  2.1     Agreement and Plan of Merger, dated as of November
           21, 1997, by and among American Tower Systems
           Corporation ("ATS"), American Tower Systems, Inc.,
           a Delaware corporation ("ATSI"), Gearon & Co.,
           Inc., a Georgia corporation ("Gearon") and J.
           Michael Gearon, Jr. (the "Gearon Stockholder").
           (Schedules and Exhibits omitted)..................   (*2.1)
  2.2     Amendment No. 1 to Agreement and Plan of Merger,
           dated as of January 22, 1998, among ATS, American
           Tower Systems (Delaware), Inc., a Delaware
           corporation (formerly known as American Tower
           Systems, Inc.), Gearon and the Gearon
           Stockholder.......................................   (*2.2)
  2.3     Agreement and Plan of Merger, dated as of December
           12, 1997, by and among ATS and American Tower
           Corporation, a Delaware corporation. (Schedules
           and Exhibits omitted).............................   (*2.3)
  2.4     Agreement and Plan of Merger, dated as of November
           16, 1998, by and among ATC, American Towers, Inc.,
           a wholly owned subsidiary of ATC and a Delaware
           corporation ("ATI"), and OmniAmerica, Inc., a
           Delaware corporation (Schedules and Exhibits
           omitted)..........................................   (**2.1)
  2.5     Agreement and Plan of Merger, dated as of November
           16, 1998, by and among ATC, ATI and TeleCom
           Towers, L.L.C., a Delaware limited liability
           company ("TeleCom") (Schedules and Exhibits
           omitted)..........................................   (**2.2)
  2.6     Amended and Restated Agreement and Plan of Merger,
           dated as of December 18, 1998, by and among ATC,
           ATI, ATC Merger Corporation, a Delaware
           corporation ("ATMC"), and TeleCom (Schedules and
           Exhibits omitted) ................................   (***2.1)
  2.7     Amendment to the Amended and Restated Agreement and
           Plan of Merger, dated as of December 23, 1998, by
           and among ATC, ATI, ATMC, and TeleCom.............   (***2.2)
  3(i).1  Restated Certificate of Incorporation of ATC, as
           filed with the Secretary of State of the State of
           Delaware on June 5, 1998..........................   (++3(i).1)
  3(i).2  Certificate of Designation relating to Exchange
           Pay-In-Kind Preferred Stock as filed with the
           Secretary of State of the State of Delaware on
           June 4, 1998......................................   (++3(i).2)
  3(i).3  Certificate of Designation relating to Series A
           Redeemable Pay-In-Kind Preferred Stock, as filed
           with the Secretary of State of the State of
           Delaware on June 4, 1998..........................   (++3(i).3)
  3(ii).1 By-Laws of ATC.....................................   (++3(ii).1)
  5       Opinion of Sullivan & Worcester LLP................   Filed herewith
                                                                as Exhibit 5
 10.1     Parent Loan Agreement, dated as of June 16, 1998,
           by and among ATC, Toronto Dominion (Texas), Inc.,
           as Administrative Agent, and the Lenders parties
           thereto (the "Parent Loan Agreement").............   (++10.1)
 10.1A    First Amendment to the Parent Loan Agreement, dated
           as of October 30, 1998, by and among ATC, Toronto
           Dominion (Texas), Inc., as Administrative Agent,
           and the Lenders Parties thereo....................   (+++10.1A)
</TABLE>
 
                                      II-3
<PAGE>
 
<TABLE>
<CAPTION>
 Exhibit                                                        Exhibit  File
   No.                  Description of Document                      No.
 -------                -----------------------                 --------------
 <C>     <S>                                                    <C>
 10.2A   ATS Facility A Loan Agreement, dated as of June 16,
          1998 among American Tower Systems, L.P. ("ATSLP")
          and ATSI, as borrowers, and Toronto Dominion
          (Texas), Inc., as Administrative Agent, and the
          Banks parties thereto (the "Facility A Loan
          Agreement")........................................   (++10.2A)
 10.2B   ATS Facility B Loan Agreement, dated as of June 16,
          1998, by and among ATSLP and ATSI, as borrowers,
          and Toronto Dominion (Texas), Inc., and the Banks
          parties thereto (the "Facility B Loan Agreement")..   (++10.2B)
 10.3    First Amendment to the ATS Facility A Loan
          Agreement, dated as of October 30, 1998 among ATSLP
          and ATI, as borrowers, and Toronto Dominion
          (Texas), Inc., as Administrative Agent, and the
          Banks parties thereto..............................   (+++10.3)
 10.3A   Second Amendment to the ATS Facility A Loan
          Agreement, dated February 8, 1999, among ATSLP and
          ATI, as borrowers, and Toronto Dominion (Texas),      Filed herewith
          Inc., as Administrative Agent, and the Banks          as Exhibit
          parties thereto....................................   10.3A
 10.4    First Amendment to the ATS Facility B Loan
          Agreement, dated as of October 30, 1998 among ATSLP
          and ATI, as borrowers, and Toronto Dominion
          (Texas), Inc., as Administrative Agent, and the
          Banks parties thereto..............................   (+++10.4)
 10.4A   Second Amendment to the ATS Facility B Loan
          Agreement, dated February 8, 1997, among ATSLP and
          ATI, as borrowers, and Toronto Dominion (Texas),      Filed herewith
          Inc., as Administrative Agent, and the Banks          as Exhibit
          parties thereto....................................   10.4A
 10.5    Asset Purchase Agreement, dated as of February 5,
          1997, by and between ATSI and Meridian Radio Sites,
          a California general partnership ("Meridian
          Radio"). (Schedules and Exhibits omitted)..........   (*10.5)
 10.6    First Amendment to Asset Purchase Agreement, dated
          as of February 10, 1997, by and between ATSI and
          Meridian Radio.....................................   (*10.6)
 10.7    Second Amendment to Asset Purchase Agreement, dated
          as of June 24, 1997, by and between ATSI and
          Meridian Radio.....................................   (*10.7)
 10.8    Asset Purchase Agreement, dated as of February 5,
          1997, by and between ATSI and Meridian Sales and
          Services Company, a California corporation
          ("Meridian Sales"). (Schedules and Exhibits
          omitted)...........................................   (*10.8)
 10.9    First Amendment to Asset Purchase Agreement, dated
          as of February 10, 1997, by and between ATSI and
          Meridian Sales.....................................   (*10.9)
 10.10   Second Amendment to Asset Purchase Agreement, dated
          as of June 24, 1997, by and between ATSI and
          Meridian Sales.....................................   (*10.10)
 10.11   Asset Purchase Agreement, dated as of February 5,
          1997, by and between ATSI and Meridian
          Communications North, a California general
          partnership ("Meridian North"). (Schedules and
          Exhibits omitted)..................................   (*10.11)
 10.12   First Amendment to Asset Purchase Agreement, dated
          as of February 10, 1997, by and between ATSI and
          Meridian North.....................................   (*10.12)
 10.13   Second Amendment to Asset Purchase Agreement, dated
          as of June 24, 1997, by and between ATSI and
          Meridian North.....................................   (*10.13)
 10.14   Asset Purchase Agreement, dated as of July 8, 1997,
          by and between ATSI and Diablo Communications,
          Inc., a California corporation. (Schedules and
          Exhibits omitted)..................................   (*10.20)
 10.15   Asset Purchase Agreement, dated as of July 8, 1997,
          by and between ATSI and Diablo Communications of
          Southern California, Inc., a California
          corporation. (Schedules and Exhibits omitted)......   (*10.21)
 10.16   Asset Purchase Agreement, dated as of July 8, 1997,
          by and between ATS and Suburban Cable T.V. Co......   (*10.22)
</TABLE>
 
 
                                      II-4
<PAGE>
 
<TABLE>
<CAPTION>
 Exhibit                                                        Exhibit  File
   No.                  Description of Document                      No.
 -------                -----------------------                 --------------
 <C>     <S>                                                    <C>
 10.17   Stock Purchase Agreement, date as of September 30,
          1997, by and between ATSI, OPM-USA-INC., a Florida
          corporation ("OPM"), and the Stockholders of OPM.
          (Schedules and Exhibits omitted)...................   (*10.24)
 10.18   Asset Purchase Agreement, dated as of October 4,
          1997, by and between ATSI and Tucson Communications
          Company, L.P., a California limited partnership.
          (Schedules and Exhibits omitted)...................   (*10.25)
 10.19   American Tower Systems Corporation 1997 Stock Option
          Plan, dated as of November 5, 1997, as amended and
          restated on April 28, 1998.........................   (+10.26)
 10.20   American Tower Systems Corporation Stock Purchase
          Agreement, dated as of January 8, 1998, by and
          among ATS and the Purchasers.......................   (*10.27)
 10.21   Employment Agreement, dated as of January 22, 1988,
          by and between ATSI and J. Michael Gearon, Jr......   (*10.28)
 10.22   Letter of Agreement, dated as of April 13, 1998, by
          and between ATC and Douglas Wiest..................   (+++10.22)
 10.23   ARS-ATS Separation Agreement, dated as of June 4,
          1998 by and among American Radio Systems
          Corporation, a Delaware Corporation, ATS, and CBS
          Corporation........................................   (++10.30)
 10.24   Registration Rights Agreement, dated as of January
          22, 1998, by and among ATC and each of the Parties    Incorporated
          named therein......................................   by reference
                                                                to Exhibit
                                                                10.3 from the
                                                                Company's
                                                                Quarterly
                                                                Report on Form
                                                                10-Q (File No.
                                                                001-14194)
                                                                dated August
                                                                14, 1998
 10.24A     Amendment to Registration Rights Agreement, dated   Filed herewith
         February 4, 1999....................................   as Exhibit
                                                                10.24A
 10.25   Stock Purchase Agreement, dated as of February 4,
          1999, by and between ATC and Credit Suisse First      Filed herewith
          Boston Corporation.................................   as Exhibit
                                                                10.25
 10.26   Registration Rights Agreement, dated as of February
          4, 1999, by and among ATC and Credit Suisse First     Filed herewith
          Boston Corporation.................................   as Exhibit
                                                                10.26
 21      Subsidiaries of ATC.................................   (+++21)
 23.0    Consent of Sullivan & Worcester LLP.................   Contained in
                                                                the opinion of
                                                                Sullivan &
                                                                Worcester LLP
                                                                filed as part
                                                                of Exhibit 5
                                                                (+)
 23.1    Independent Auditors' Consent--Deloitte & Touche       Exhibit 23.1
          LLP................................................   (+)
 23.2    Consent of Pressman Ciocca Smith LLP................   Exhibit 23.2
                                                                (+)
 23.3    Consent of Rooney, Ida, Nolt & Ahern................   Exhibit 23.3
                                                                (+)
 23.4    Consent of Ernst & Young LLP........................   Exhibit 23.4
                                                                (+)
</TABLE>
 
                                      II-5
<PAGE>
 
<TABLE>
<CAPTION>
 Exhibit                                                         Exhibit  File
   No.                   Description of Document                      No.
 -------                 -----------------------                 --------------
 <C>     <S>                                                     <C>
 23.5    Consent of KPMG LLP ..................................  Exhibit 23.5
                                                                 (+)
 23.6    Consent of Bill Mitts, Inc............................  Exhibit 23.6
                                                                 (+)
 23.7    Consent of Arthur Andersen LLP........................  Exhibit 23.7
                                                                 (+)
 23.8    Consent of Gollob, Morgan, Peddy & Co., P.C...........  Exhibit 23.8
                                                                 (+)
 23.9    Consent of KPMG LLP...................................  Exhibit 23.9
                                                                 (+)
 23.10   Consent of Ernst & Young LLP..........................  Exhibit 23.10
                                                                 (+)
 23.11   Consent of KPMG LLP...................................  Exhibit 23.11
                                                                 (+)
 24      Power of Attorney.....................................  Filed as page
                                                                 II-7 of the
                                                                 Registration
                                                                 Statement (+)
 99.1    Consent of Director Nominee, Jack D. Furst............  Exhibit 99.1
                                                                 (+)
 99.2    Consent of Director Nominee, Dean H. Eisner...........  Exhibit 99.2
                                                                 (+)
</TABLE>
 
Item 17. Undertakings.
 
  The undersigned registrant hereby undertakes that:
 
  (1) To file, during any period in which offers or sales are being made, a
post-effective amendment to this registration statement:
 
  (i) To include any prospectus required by Section 10(a)(3) of the Securities
Act of 1933;
 
  (ii) To reflect in the prospectus any facts or events arising after the
effective date of the registration statement (or the most recent post-effective
amendment thereof) which, individually or in the aggregate, represent a
fundamental change in the information set forth in the registration statement.
Notwithstanding the foregoing, any increase or decrease in volume of securities
offered (if the total dollar value of securities offered would not exceed that
which was registered) and any deviation from the low or high end of the
estimated maximum offering range may be reflected in the form of prospectus
filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the
changes in volume and price represent no more than 20 percent change in the
maximum aggregate offering price set forth in the "Calculation of Registration
Fee" table in the effective registration statement.
 
  (iii) To include any material information with respect to the plan of
distribution not previously disclosed in the registration statement or any
material change to such information in the registration statement;
 
  (2) That, for the purpose of determining any liability under the Securities
Act of 1933, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.
 
 
                                      II-6
<PAGE>
 
  (3) To remove from registration by means of a post-effective amendment any of
the securities being registered which remain unsold at the termination of the
offering.
 
  (4) For the purpose of determining any liability under the Securities Act,
each post-effective amendment that contains a form of prospectus shall be
deemed to be a new registration statement relating to the securities offered
therein, and the offering of such securities at the time shall be deemed to be
the initial bona fide offering thereof.
 
                                      II-7
<PAGE>
 
                                   SIGNATURES
 
  Pursuant to the requirements of the Securities Act, the Company certifies
that it has reasonable grounds to believe that it meets all of the requirements
for filing on Form S-1 and has duly caused this registration statement to be
signed on its behalf by the undersigned, thereunto duly authorized, in the City
of Boston, Commonwealth of Massachusetts, on the 8th day of February, 1999.
 
                                          AMERICAN TOWER CORPORATION
 
                                          By: /s/ Steven B. Dodge *
                                          -------------------------------------
                                                     Steven B. Dodge
                                                   Chairman of the Board,
                                               President and Chief Executive
                                                          Officer
 
<TABLE>
<CAPTION>
             Signature                           Title                   Date
             ---------                           -----                   ----
<S>                                  <C>                           <C>
       /s/ Steven B. Dodge *         Chairman, President, Chief    February 8, 1999
____________________________________  Executive Officer and
          Steven B. Dodge             Director
        /s/ Joseph L. Winn *         Chief Financial Officer and   February 8, 1999
____________________________________  Treasurer
           Joseph L. Winn
      /s/ Justin D. Benincasa        Vice President and Corporate  February 8, 1999
____________________________________  Controller
        Justin D. Benincasa
individually and as attorney-in-fact
 
         /s/ Alan L. Box *           Executive Vice President and  February 8, 1999
____________________________________  Director
            Alan L. Box
 
                                     Director                      February 8, 1999
____________________________________
         Arnold L. Chavkin
 
    /s/ J. Michael Gearon, Jr. *     Executive Vice President and  February 8, 1999
____________________________________  Director
       J. Michael Gearon Jr.
 
                                     Director                      February 8, 1999
____________________________________
            Randall Mays
 
 
         /s/ Fred Lummis *           Director                      February 8, 1999
____________________________________
            Fred Lummis
 
       /s/ Thomas H. Stoner *        Director                      February 8, 1999
____________________________________
          Thomas H. Stoner
 
                                     Director                      February 8, 1999
____________________________________
         Maggie Wilderotter
</TABLE>
 
                                      II-8
<PAGE>
 
                                                                     SCHEDULE II
 
                  AMERICAN TOWER CORPORATION AND SUBSIDIARIES
 
                       VALUATION AND QUALIFYING ACCOUNTS
 
       Period from July 17, 1995 (Incorporation) to December 31, 1995 and
                   the Years Ended December 31, 1996 and 1997
 
<TABLE>
<CAPTION>
                            Column A   Column B  Column C  Column D   Column E
                           ---------- ---------- -------- ---------- ----------
                           Balance at Charged to Charged             Balance at
                           Beginning   Cost and  to Other              End of
       Description         of Period   Expenses  Accounts Deductions   Period
       -----------         ---------- ---------- -------- ---------- ----------
<S>                        <C>        <C>        <C>      <C>        <C>
Allowance for Doubtful
 Accounts:
 Period from July 17,
  1995 to December 31,
  1995...................   $   --     $    --     $--     $   --     $    --
 Year Ended December 31,
  1996...................   $   --     $ 47,044    $--     $   --     $ 47,044
 Year Ended December 31,
  1997...................   $47,044    $124,350    $--     $46,310    $125,084
</TABLE>
 
                                      S-I
<PAGE>
 
                                 EXHIBIT INDEX
 
  Listed below are the exhibits which are filed as part of this registration
statement (according to the number assigned to them in Item 601 of Regulation
S-K). Each exhibit marked by a (*) is incorporated by reference to ATC's
Registration Statement on Form S-4 (File No. 333-46025) filed on February 10,
1998. Each exhibit marked by a (+) is incorporated by reference to ATC's
Registration Statement on Form S-1 (File No. 333-50111) filed on May 8, 1998.
Each exhibit marked by a (++) is incorporated by reference to the filing of
Amendment No. 2 to ATC's Registration Statement on Form S-1 (No. 333-52481)
filed on June 30, 1998. Each exhibit marked by a (**) is incorporated by
reference to the filing of ATC's Current Report on Form 8-K filed on November
30, 1998. Each exhibit marked by a (***) is incorporated by reference to the
filing of ATC's Current Report on Form 8-K filed on January 8, 1999. Exhibit
numbers in parenthesis refer to the exhibit number in the applicable filing.
Each exhibit marked by a (+++) is incorporated by reference to ATC's
Registration Statement on Form S-4 (File No. 333-76083) filed on January 15,
1999. Each Exhibit marked by a (++++) is incorporated by reference from
Registration Statement (333-70881) on January 21, 1999. Each exhibit marked by
a (****) is incorporated by reference to the filing of Amendment No. 1 to ATC's
Registration Statement on Form S-1 (File No. 333-70881) filed on February 3,
1999. Each exhibit marked by a (+) is incorporated by reference from the
initial filing of this Registration Statement (No. 333-71939) on February 5,
1999.
 
<TABLE>
<CAPTION>
 Exhibit                                                         Exhibit  File
   No.                  Description of Document                       No.
 -------                -----------------------                  --------------
 <C>     <S>                                                     <C>
  2.1    Agreement and Plan of Merger, dated as of November
          21, 1997, by and among American Tower Systems
          Corporation ("ATS"), American Tower Systems, Inc., a
          Delaware corporation ("ATSI"), Gearon & Co., Inc., a
          Georgia corporation ("Gearon") and J. Michael
          Gearon, Jr. (the "Gearon Stockholder"). (Schedules
          and Exhibits omitted)...............................   (*2.1)
  2.2    Amendment No. 1 to Agreement and Plan of Merger,
          dated as of January 22, 1998, among ATS, American
          Tower Systems (Delaware), Inc., a Delaware
          corporation (formerly known as American Tower
          Systems, Inc.), Gearon and the Gearon Stockholder...   (*2.2)
  2.3    Agreement and Plan of Merger, dated as of December
          12, 1997, by and among ATS and American Tower
          Corporation, a Delaware corporation. (Schedules and
          Exhibits omitted)...................................   (*2.3)
  2.4    Agreement and Plan of Merger, dated as of November
          16, 1998, by and among ATC, American Towers, Inc., a
          wholly owned subsidiary of ATC and a Delaware
          corporation ("ATI"), and OmniAmerica, Inc., a
          Delaware corporation (Schedules and Exhibits
          omitted)............................................   (**2.1)
  2.5    Agreement and Plan of Merger, dated as of November
          16, 1998, by and among ATC, ATI and TeleCom Towers,
          L.L.C., a Delaware limited liability company
          ("TeleCom") (Schedules and Exhibits omitted)........   (**2.2)
  2.6    Amended and Restated Agreement and Plan of Merger,
          dated as of December 18, 1998, by and among ATC,
          ATI, ATC Merger Corporation, a Delaware corporation
          ("ATMC"), and TeleCom (Schedules and Exhibits
          omitted) ...........................................   (***2.1)
  2.7    Amendment to the Amended and Restated Agreement and
          Plan of Merger, dated as of December 23, 1998, by
          and among ATC, ATI, ATMC, and TeleCom...............   (***2.2)
  3(i).1 Restated Certificate of Incorporation of ATC, as
          filed with the Secretary of State of the State of
          Delaware on June 5, 1998............................   (++3(i).1)
  3(i).2 Certificate of Designation relating to Exchange Pay-
          In-Kind Preferred Stock as filed with the Secretary
          of State of the State of Delaware on June 4, 1998...   (++3(i).2)
  3(i).3 Certificate of Designation relating to Series A
          Redeemable Pay-In-Kind Preferred Stock, as filed
          with the Secretary of State of the State of Delaware
          on June 4, 1998.....................................   (++3(i).3)
</TABLE>
<PAGE>
 
<TABLE>
<CAPTION>
 Exhibit                                                         Exhibit  File
   No.                   Description of Document                      No.
 -------                 -----------------------                 --------------
 <C>      <S>                                                    <C>
  3(ii).1 By-Laws of ATC......................................   (++3(ii).1)
  5       Opinion of Sullivan & Worcester LLP.................   Filed herewith
                                                                 as Exhibit 5
 10.1     Parent Loan Agreement, dated as of June 16, 1998, by
           and among ATC, Toronto Dominion (Texas), Inc., as
           Administrative Agent, and the Lenders parties
           thereto (the "Parent Loan Agreement")..............   (++10.1)
 10.1A    First Amendment to the Parent Loan Agreement, dated
           as of October 30, 1998, by and among ATC, Toronto
           Dominion (Texas), Inc., as Administrative Agent,
           and the Lenders Parties thereo.....................   (+++10.1A)
 10.2A    ATS Facility A Loan Agreement, dated as of June 16,
           1998 among American Tower Systems, L.P. ("ATSLP")
           and ATSI, as borrowers, and Toronto Dominion
           (Texas), Inc., as Administrative Agent, and the
           Banks parties thereto (the "Facility A Loan
           Agreement")........................................   (++10.2A)
 10.2B    ATS Facility B Loan Agreement, dated as of June 16,
           1998, by and among ATSLP and ATSI, as borrowers,
           and Toronto Dominion (Texas), Inc., and the Banks
           parties thereto (the "Facility B Loan Agreement")..   (++10.2B)
 10.3     First Amendment to the ATS Facility A Loan
           Agreement, dated as of October 30, 1998 among ATSLP
           and ATI, as borrowers, and Toronto Dominion
           (Texas), Inc., as Administrative Agent, and the
           Banks parties thereto..............................   (+++10.3)
 10.3A    Second Amendment to the ATS Facility A Loan
           Agreement, dated February 8, 1999, among ATSLP and
           ATI, as borrowers and Toronto Dominion (Texas),       Filed herewith
           Inc., as Administrative Agent, and the Banks          as Exhibit
           parties thereto....................................   10.3A
 10.4     First Amendment to the ATS Facility B Loan
           Agreement, dated as of October 30, 1998 among ATSLP
           and ATI, as borrowers, and Toronto Dominion
           (Texas), Inc., as Administrative Agent, and the
           Banks parties thereto..............................   (+++10.4)
 10.4A    Second Amendment to the ATS Facility B Loan
           Agreement, dated February 8, 1997, among ATSLP and
           ATI, as borrowers, and Toronto Dominion (Texas),      Filed herewith
           Inc., as Administrative Agent, and the Banks          as Exhibit
           parties thereto ...................................   10.4A
 10.5     Asset Purchase Agreement, dated as of February 5,
           1997, by and between ATSI and Meridian Radio Sites,
           a California general partnership ("Meridian
           Radio"). (Schedules and Exhibits omitted)..........   (*10.5)
 10.6     First Amendment to Asset Purchase Agreement, dated
           as of February 10, 1997, by and between ATSI and
           Meridian Radio.....................................   (*10.6)
 10.7     Second Amendment to Asset Purchase Agreement, dated
           as of June 24, 1997, by and between ATSI and
           Meridian Radio.....................................   (*10.7)
 10.8     Asset Purchase Agreement, dated as of February 5,
           1997, by and between ATSI and Meridian Sales and
           Services Company, a California corporation
           ("Meridian Sales"). (Schedules and Exhibits
           omitted)...........................................   (*10.8)
 10.9     First Amendment to Asset Purchase Agreement, dated
           as of February 10, 1997, by and between ATSI and
           Meridian Sales.....................................   (*10.9)
 10.10    Second Amendment to Asset Purchase Agreement, dated
           as of June 24, 1997, by and between ATSI and
           Meridian Sales.....................................   (*10.10)
 10.11    Asset Purchase Agreement, dated as of February 5,
           1997, by and between ATSI and Meridian
           Communications North, a California general
           partnership ("Meridian North"). (Schedules and
           Exhibits omitted)..................................   (*10.11)
</TABLE>
<PAGE>
 
<TABLE>
<CAPTION>
 Exhibit                                                         Exhibit  File
   No.                  Description of Document                       No.
 -------                -----------------------                  --------------
 <C>     <S>                                                     <C>
 10.12   First Amendment to Asset Purchase Agreement, dated as
          of February 10, 1997, by and between ATSI and
          Meridian North......................................   (*10.12)
 10.13   Second Amendment to Asset Purchase Agreement, dated
          as of June 24, 1997, by and between ATSI and
          Meridian North......................................   (*10.13)
 10.14   Asset Purchase Agreement, dated as of July 8, 1997,
          by and between ATSI and Diablo Communications, Inc.,
          a California corporation. (Schedules and Exhibits
          omitted)............................................   (*10.20)
 10.15   Asset Purchase Agreement, dated as of July 8, 1997,
          by and between ATSI and Diablo Communications of
          Southern California, Inc., a California corporation.
          (Schedules and Exhibits omitted)....................   (*10.21)
 10.16   Asset Purchase Agreement, dated as of July 8, 1997,
          by and between ATS and Suburban Cable T.V. Co.......   (*10.22)
 10.17   Stock Purchase Agreement, date as of September 30,
          1997, by and between ATSI, OPM-USA-INC., a Florida
          corporation ("OPM"), and the Stockholders of OPM.
          (Schedules and Exhibits omitted)....................   (*10.24)
 10.18   Asset Purchase Agreement, dated as of October 4,
          1997, by and between ATSI and Tucson Communications
          Company, L.P., a California limited partnership.
          (Schedules and Exhibits omitted)....................   (*10.25)
 10.19   American Tower Systems Corporation 1997 Stock Option
          Plan, dated as of November 5, 1997, as amended and
          restated on April 28, 1998..........................   (+10.26)
 10.20   American Tower Systems Corporation Stock Purchase
          Agreement, dated as of January 8, 1998, by and among
          ATS and the Purchasers..............................   (*10.27)
 10.21   Employment Agreement, dated as of January 22, 1988,
          by and between ATSI and J. Michael Gearon, Jr.......   (*10.28)
 10.22   Letter of Agreement, dated as of April 13, 1998, by
          and between ATC and Douglas Wiest...................   (+++10.22)
 10.23   ARS-ATS Separation Agreement, dated as of June 4,
          1998 by and among American Radio Systems
          Corporation, a Delaware Corporation ("ARS"), ATS,
          and CBS Corporation.................................   (++10.30)
 10.24   Registration Rights Agreement, dated as of January
          22, 1998, by and among ATC and each of the Parties     Incorporated
          named therein.......................................   by reference
                                                                 to Exhibit
                                                                 10.3 from the
                                                                 Company's
                                                                 Quarterly
                                                                 Report on Form
                                                                 10-Q (File No.
                                                                 001-14194)
                                                                 dated August
                                                                 14, 1998
                                                                 Filed herewith
             Amendment to Registration Rights Agreement, dated   as Exhibit
 10.24A  February 4, 1999.....................................   10.24A
 10.25   Stock Purchase Agreement, dated as of February 4,       Filed herewith
          1999, by and among ATC and Credit Suisse First         as Exhibit
          Boston Corporation..................................   10.25
 10.26   Registration Rights Agreement, dated as of February     Filed herewith
          4, 1999, by and among ATC and Credit Suisse First      as Exhibit
          Boston Corporation..................................   10.26
 21      Subsidiaries of ATC..................................   (+++21)
</TABLE>
<PAGE>
 
<TABLE>
<CAPTION>
 Exhibit                                                        Exhibit  File
   No.                  Description of Document                      No.
 -------                -----------------------                 --------------
 <C>     <S>                                                    <C>
 23.0    Consent of Sullivan & Worcester LLP..................  Contained in
                                                                the opinion of
                                                                Sullivan &
                                                                Worcester LLP
                                                                filed herewith
                                                                as part of
                                                                Exhibit 5 (+)
 23.1    Independent Auditors' Consent--Deloitte & Touche       Exhibit 23.1
          LLP.................................................  (+)
 23.2    Consent of Pressman Ciocca Smith LLP.................  Exhibit
                                                                23.2 (+)
 23.3    Consent of Rooney, Ida, Nolt & Ahern.................  Exhibit 23.3
                                                                (+)
 23.4    Consent of Ernst & Young LLP.........................  Exhibit
                                                                23.4 (+)
 23.5    Consent of KPMG LLP .................................  Exhibit
                                                                23.5 (+)
 23.6    Consent of Bill Mitts, Inc...........................  Exhibit
                                                                23.6 (+)
 23.7    Consent of Arthur Andersen LLP.......................  Exhibit
                                                                23.7 (+)
 23.8    Consent of Gollob, Morgan, Peddy & Co., P.C..........  Exhibit
                                                                23.8 (+)
 23.9    Consent of KPMG LLP..................................  Exhibit
                                                                23.9 (+)
 23.10   Consent of Ernst & Young LLP.........................  Exhibit
                                                                23.10 (+)
 23.11   Consent of KPMG LLP..................................  Exhibit
                                                                23.11 (+)
 24      Power of Attorney....................................  Filed as page
                                                                II-7 of the
                                                                Registration
                                                                Statement (+)
 99.1    Consent of Director Nominee, Jack D. Furst...........  Exhibit
                                                                99.1 (+)
 99.2    Consent of Director Nominee, Dean H. Eisner..........  Exhibit
                                                                99.2 (+)
</TABLE>

<PAGE>

                                                                   EXHIBIT 10.3A
 
                                                                  EXECUTION COPY
                                                                                

                          SECOND AMENDMENT AND WAIVER
                                      TO
                         ATS FACILITY A LOAN AGREEMENT


     THIS SECOND AMENDMENT AND WAIVER TO ATS FACILITY A LOAN AGREEMENT (this
"Amendment and Waiver"), dated as of the 8th day of February, 1999 (the
 --------------------                                                  
"Amendment and Waiver Date"), by and among AMERICAN TOWER, L.P. (formerly known
 -------------------------                                                     
as American Tower Systems, L.P.), a Delaware limited partnership, and AMERICAN
TOWERS, INC. (formerly known as American Tower Systems (Delaware), Inc.), a
Delaware corporation (collectively, the "Borrower"), the FINANCIAL INSTITUTIONS
                                         --------                              
SIGNATORY HERETO and TORONTO DOMINION (TEXAS), INC., as administrative agent
(the "Administrative Agent") for the Banks (as defined in the Loan Agreement
      --------------------                                                  
defined below);


                             W I T N E S S E T H:
                              - - - - - - - - - -


     WHEREAS, the Borrower, the Banks and the Administrative Agent are parties
to that certain ATS Facility A Loan Agreement dated as of June 16, 1998, as
amended by that certain First Amendment to ATS Facility A Loan Agreement dated
as of October 30, 1998 (as amended, modified, supplemented and restated from
time to time, the "Loan Agreement"); and
                   --------------       

     WHEREAS, the Borrower has informed the Administrative Agent that American
Tower Corporation, a Delaware corporation and the parent company of the Borrower
(the "Parent") will issue an estimated 25,700,000 shares of Capital Stock in a
public sale and 500,000 shares of Capital Stock in a private sale, both to be
consummated in February, 1999 (the actual issuance being referred to hereinafter
as the "1999 Parent Equity Issuance"); and

     WHEREAS, the Borrower has informed Administrative Agent that it would like
to receive Facility A Capital Raise Proceeds in connection with the 1999 Parent
Equity Issuance without (a) making certain repayments of the Loans, as would
otherwise be required pursuant to Section 2.7(b)(v) of the Loan Agreement (the
"Mandatory Repayment") or (b) having the Revolving Loan Commitment reduced, as
would otherwise be required pursuant to Section 2.5(d) of the Loan Agreement,
upon and in an amount equal to any repayment of the Revolving Loans required
under Section 2.7(b)(v) of the Loan Agreement (the "Mandatory Reduction"), in
each case, upon receipt by the Parent, the Borrower or any Restricted Subsidiary
of any such Facility A Capital Raise Proceeds; and
<PAGE>
 
     WHEREAS, the Borrower has requested that the Banks waive, solely in
connection with the 1999 Parent Equity Issuance, each of (i) the Mandatory
Reduction, (ii) the Mandatory Repayment and (iii) any Default or Event of
Default under the Facility A Loan Agreement which would otherwise occur if the
Mandatory Repayment is not made; and

     WHEREAS, the Borrower and the Banks have agreed to (a) waive, solely in
connection with the 1999 Parent Equity Issuance, (i) the Mandatory Reduction,
(iii) the Mandatory Repayment and (ii) any Default or Event of Default under the
Facility A Loan Agreement which would otherwise occur if the Mandatory Repayment
is made not made, and (b) amend Section 2.3 (f) of the Loan Agreement to change
the Applicable Margin set forth therein, in each case, on the terms and
conditions set forth herein;

     NOW, THEREFORE, in consideration of the premises set forth above, the
covenants and agreements hereinafter set forth, and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto agree that all capitalized terms used herein shall have the
meanings ascribed thereto in the Loan Agreement, and further agree as follows:

     1.   Waiver In Connection with the Equity Issuance.   This Amendment and
          ---------------------------------------------                      
Waiver, when executed by each of the Banks and accepted by the Borrower, will,
subject to the terms and conditions hereof, reflect the Banks' waiver, solely in
connection with the 1999 Parent Equity issuance of (i) the Mandatory Reduction,
(ii) the Mandatory Repayment, and (iii) any Default or Event of Default under
the Loan Agreement which would otherwise arise as a direct or indirect result of
the failure to make the Mandatory Repayment, in each case, as would otherwise be
required under the Loan Agreement as a direct or indirect result of receipt by
the Parent, the Borrower or any Restricted Subsidiary of any Facility A Capital
Raise Proceeds in connection with the 1999 Parent Equity Issuance.

     2.   Amendment to Article 2.  Section 2.3(f) of the Loan Agreement, 
          ----------------------                                        
Interest: Applicable Margin, is hereby amended by deleting the first sentence
- ---------------------------                                                  
immediately following the Applicable Margin Ratio grid set forth in such section
in its entirety and by substituting in lieu thereof the following:

     "Notwithstanding the foregoing, the LIBOR Advance Applicable Margin shall
     not be less than, (i) prior to and including June 30, 1999, two percent
     (2.000%), and (ii) from July 1, 1999 through and including December 31,
     1999, one and three quarters of one percent (1.750%)."

     3.   No Other Amendment or Waiver.  Except for the amendments set forth 
          ----------------------------                                
above, the text of the Loan Agreement and all other Loan Documents shall remain
unchanged and in full force and effect. No waiver by the Administrative Agent or
the Banks under the Loan

                                      -2-
<PAGE>
 
Agreement or any other Loan Document is granted or intended except as expressly
set forth herein, and the Administrative Agent and the Banks expressly reserve
the right to require strict compliance in all other respects (whether or not in
connection with any Requests for Advance). Except as set forth herein, the
amendment agreed to herein shall not constitute a modification of the Loan
Agreement or any of the other Loan Documents, or a course of dealing with the
Administrative Agent and the Banks at variance with the Loan Agreement or any of
the other Loan Documents, such as to require further notice by the
Administrative Agent, the Banks or the Majority Banks to require strict
compliance with the terms of the Loan Agreement and the other Loan Documents in
the future.

     4.   Representations and Warranties.  The Borrower hereby represents and
          ------------------------------                                 
warrants in favor of each of the Administrative Agent and each Bank that the
representations and warranties contained in Section 4.1 of the Loan Agreement
and contained in the other Loan Documents remain true and correct as of the date
hereof, both before and after giving effect to this Amendment and Waiver, except
to the extent previously fulfilled in accordance with the terms of the Loan
Agreement or such other Loan Document, as applicable, or to the extent relating
specifically to the Agreement Date. No Default now exists or will be caused
hereby.

     5.   Condition Precedent.  The effectiveness of this Amendment and Waiver
          -------------------                                          
is subject to the receipt by the Administrative Agent of counterparts hereof
executed by the Banks and the Borrower and of all documents, instruments,
consents or items which the Administrative Agent shall deem appropriate in
connection herewith.

     6.   Loan Documents.  This document shall be deemed to be a Loan Document
          --------------                                             
for all purposes under the Loan Agreement and the other Loan Documents.

     7.   Counterparts.  This Amendment and Waiver may be executed in any
          ------------                                                   
number of counterparts, each of which shall be deemed to be an original, but all
such separate counterparts shall together constitute but one and the same
instrument.

     8.   Governing Law.  This Amendment and Waiver shall be construed in
          -------------                                                  
accordance with and governed by the laws of the State of New York.

     9.   Severability.  Any provision of this Amendment and Waiver which is
          ------------                                                   
prohibited or unenforceable shall be ineffective to the extent of such
prohibition or unenforceability without invalidating the remaining provisions
hereof in that jurisdiction or affecting the validity or enforceability of such
provision in any other jurisdiction.

             [THE REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]

                                      -3-
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto have executed this Amendment and
Waiver or caused it to be executed by their duly authorized officers, all as of
the day and year first above written.


BORROWERS:                    AMERICAN TOWER, L.P. (formerly known as 
                              American Tower Systems, L.P.)

                              By ATC GP INC. (formerly known as ATSC GP Inc.),
                              its General Partner


                              By:_____________________________________________

                                   Title:_____________________________________


                              AMERICAN TOWERS, INC. (formerly known as 
                              American Tower Systems (Delaware), Inc.)


                              By:_____________________________________________

                                   Title:_____________________________________


ADMINISTRATIVE AGENT
AND BANKS:                    TORONTO DOMINION (TEXAS), INC., as 
                              Administrative Agent for itself and the Banks 
                              and as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 1
<PAGE>
 
                              BANK OF AMERICA NATIONAL TRUST AND 
                              SAVINGS ASSOCIATION, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              BANK OF MONTREAL, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              THE BANK OF NEW YORK, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              THE BANK OF NOVA SCOTIA, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              BANK OF SCOTLAND, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________

                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 2
<PAGE>
 
                              BANKBOSTON, N.A., as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              BANKERS TRUST COMPANY, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              BARCLAYS BANK, PLC, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              THE CHASE MANHATTAN BANK, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              THE CIT GROUP/EQUIPMENT FINANCING, INC., as 
                              a Bank


                              By:_____________________________________________

                                   Title:_____________________________________

                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 3
<PAGE>
 
                              CITY NATIONAL BANK, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              COBANK, ACB, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              CREDIT LYONNAIS NEW YORK BRANCH, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              CREDIT SUISSE FIRST BOSTON, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              By:_____________________________________________

                                   Title:_____________________________________

                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 4
<PAGE>
 
                              CRESTAR BANK, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              DRESDNER BANK AG, NEW YORK AND GRAND 
                              CAYMAN BRANCHES, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              By:_____________________________________________

                                   Title:_____________________________________


                              FIRST NATIONAL BANK OF MARYLAND, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              FLEET NATIONAL BANK, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________

                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 5
<PAGE>
 
                              GENERAL ELECTRIC CAPITAL CORPORATION, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              THE HOWARD BANK, N.A., as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              KEY CORPORATE CAPITAL INC., as a Bank


                              By:____________________________________________

                                   Title:____________________________________


                              LEHMAN COMMERCIAL PAPER INC., as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              THE LONG-TERM CREDIT BANK OF JAPAN, LTD., 
                              NEW YORK BRANCH, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________

                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 6
<PAGE>
 
                              MELLON BANK, N.A., as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              MERCANTILE BANK NATIONAL ASSOCIATION, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              MORGAN STANLEY SENIOR FUNDING, INC., as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              NATIONAL BANK OF CANADA, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              By:_____________________________________________

                                   Title:_____________________________________

                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 7
<PAGE>
 
                              PNC BANK, NATIONAL ASSOCIATION, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              SOCIETE GENERALE, NEW YORK BRANCH, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              STATE STREET BANK AND TRUST COMPANY, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              UNION BANK OF CALIFORNIA, N.A., as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________


                              US TRUST, as a Bank


                              By:_____________________________________________

                                   Title:_____________________________________

                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 8
<PAGE>
 
                              SYNDICATED LOAN FUNDING TRUST

                              By Lehman Commercial Paper, Inc., not in its 
                              individual capacity, but solely as Asset Manager


                              By:_____________________________________________

                                   Title:_____________________________________

                                                           AMERICAN TOWERS, INC.
                                                           AMERICAN TOWER, L.P..
                    SECOND AMENDMENT AND WAIVER TO ITS FACILITY A LOAN AGREEMENT
                                                                Signature Page 9

<PAGE>

                                                                   EXHIBIT 10.4A
 
                                                                  EXECUTION COPY
                                                                                

                          SECOND AMENDMENT AND WAIVER
                                      TO
                         ATS FACILITY B LOAN AGREEMENT


     THIS SECOND AMENDMENT AND WAIVER TO ATS FACILITY B LOAN AGREEMENT (this
"Amendment and Waiver"), dated as of the 8th day of February, 1999 (the
 --------------------                                                  
"Amendment and Waiver Date"), by and among AMERICAN TOWER, L.P. (formerly known
 -------------------------                                                     
as American Tower Systems, L.P.), a Delaware limited partnership, and AMERICAN
TOWERS, INC. (formerly known as American Tower Systems (Delaware), Inc.), a
Delaware corporation (collectively, the "Borrower"), the FINANCIAL INSTITUTIONS
                                         --------                              
SIGNATORY HERETO and TORONTO DOMINION (TEXAS), INC., as administrative agent
(the "Administrative Agent") for the Banks (as defined in the Loan Agreement
      --------------------                                                  
defined below);


                             W I T N E S S E T H:
                             - - - - - - - - - -

     WHEREAS, the Borrower, the Banks and the Administrative Agent are parties
to that certain ATS Facility B Loan Agreement dated as of June 16, 1998, as
amended by that certain First Amendment to ATS Facility B Loan Agreement dated
as of October 30, 1998 (as amended, modified, supplemented and restated from
time to time, the "Loan Agreement"); and
                   --------------       

     WHEREAS, the Borrower has informed the Administrative Agent that American
Tower Corporation, a Delaware corporation and the parent company of the Borrower
(the "Parent") will issue an estimated 25,700,000 shares of Capital Stock in a
public sale and 500,000 shares of Capital Stock in a private sale, both to be
consummated in February, 1999 (the actual issuance being referred to hereinafter
as the "1999 Parent Equity Issuance"); and

     WHEREAS, the Borrower has informed Administrative Agent that it would like
to receive Facility B Capital Raise Proceeds in connection with the 1999 Parent
Equity Issuance without (a) making certain repayments of the Facility B Loan, as
would otherwise be required pursuant to Section 2.7(b)(v) of the Loan Agreement
(the "Mandatory Repayment") or (b) having the Facility B Commitment reduced, as
would otherwise be required pursuant to Section 2.5(c) of the Loan Agreement,
upon and in an amount equal to any repayment of the Facility B Loan required
under Section 2.7(b)(v) of the Loan Agreement (the "Mandatory Reduction"), in
each case, upon receipt by the Parent, the Borrower or any Restricted Subsidiary
of any such Facility B Capital Raise Proceeds; and

<PAGE>
 
     WHEREAS, the Borrower has requested that the Banks waive, solely in
connection with the 1999 Parent Equity Issuance, each of (i) the Mandatory
Reduction, (ii) the Mandatory Repayment and (iii) any Default or Event of
Default under the Facility B Loan Agreement which would otherwise occur if the
Mandatory Repayment is not made; and

     WHEREAS, the Borrower and the Banks have agreed to (a) waive, solely in
connection with the 1999 Parent Equity Issuance, (i) the Mandatory Reduction,
(iii) the Mandatory Repayment and (ii) any Default or Event of Default under the
Facility B Loan Agreement which would otherwise occur if the Mandatory Repayment
is made not made, and (b) amend Section 2.3 (f) of the Loan Agreement to change
the Applicable Margin set forth therein, in each case, on the terms and
conditions set forth herein;

     NOW, THEREFORE, in consideration of the premises set forth above, the
covenants and agreements hereinafter set forth, and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto agree that all capitalized terms used herein shall have the
meanings ascribed thereto in the Loan Agreement, and further agree as follows:

     1.  Waiver In Connection with the Equity Issuance.   This Amendment and
         ---------------------------------------------                      
Waiver, when executed by each of the Banks and accepted by the Borrower, will,
subject to the terms and conditions hereof, reflect the Banks' waiver, solely in
connection with the 1999 Parent Equity issuance of (i) the Mandatory Reduction,
(ii) the Mandatory Repayment, and (iii) any Default or Event of Default under
the Loan Agreement which would otherwise arise as a direct or indirect result of
the failure to make the Mandatory Repayment, in each case, as would otherwise be
required under the Loan Agreement as a direct or indirect result of receipt by
the Parent, the Borrower or any Restricted Subsidiary of any Facility B Capital
Raise Proceeds in connection with the 1999 Parent Equity Issuance.

     2.  Amendment to Article 2. Section 2.3(f) of the Loan Agreement, Interest:
         ----------------------                                        -------- 
Applicable Margin, is hereby amended by deleting the first sentence immediately
- -----------------
following the Applicable Margin Ratio grid set forth in such section in its
entirety and by substituting in lieu thereof the following:

     "Notwithstanding the foregoing, the LIBOR Advance Applicable Margin shall
     not be less than, (i) prior to and including June 30, 1999, two percent
     (2.000%), and (ii) from July 1, 1999 through and including December 31,
     1999, one and three quarters of one percent (1.750%)."

     3.  No Other Amendment or Waiver. Except for the amendments set forth
         ----------------------------   
above, the text of the Loan Agreement and all other Loan Documents shall remain
unchanged and in full force and effect. No waiver by the Administrative Agent or
the Banks under the Loan

                                      -2-
<PAGE>
 
Agreement or any other Loan Document is granted or intended except as expressly
set forth herein, and the Administrative Agent and the Banks expressly reserve
the right to require strict compliance in all other respects (whether or not in
connection with any Requests for Advance). Except as set forth herein, the
amendment agreed to herein shall not constitute a modification of the Loan
Agreement or any of the other Loan Documents, or a course of dealing with the
Administrative Agent and the Banks at variance with the Loan Agreement or any of
the other Loan Documents, such as to require further notice by the
Administrative Agent, the Banks or the Majority Banks to require strict
compliance with the terms of the Loan Agreement and the other Loan Documents in
the future.

     4.  Representations and Warranties. The Borrower hereby represents and
         ------------------------------
warrants in favor of each of the Administrative Agent and each Bank that the
representations and warranties contained in Section 4.1 of the Loan Agreement
and contained in the other Loan Documents remain true and correct as of the date
hereof, both before and after giving effect to this Amendment and Waiver, except
to the extent previously fulfilled in accordance with the terms of the Loan
Agreement or such other Loan Document, as applicable, or to the extent relating
specifically to the Agreement Date. No Default now exists or will be caused
hereby.

     5.  Condition Precedent. The effectiveness of this Amendment and Waiver is
         ------------------- 
subject to the receipt by the Administrative Agent of counterparts hereof
executed by the Banks and the Borrower and of all documents, instruments,
consents or items which the Administrative Agent shall deem appropriate in
connection herewith.

     6.  Loan Documents. This document shall be deemed to be a Loan Document for
         --------------
all purposes under the Loan Agreement and the other Loan Documents.

     7.  Counterparts.  This Amendment and Waiver may be executed in any
         ------------                                                   
number of counterparts, each of which shall be deemed to be an original, but all
such separate counterparts shall together constitute but one and the same
instrument.

     8.  Governing Law.  This Amendment and Waiver shall be construed in
         -------------                                                  
accordance with and governed by the laws of the State of New York.

     9.  Severability. Any provision of this Amendment and Waiver which is
         ------------
prohibited or unenforceable shall be ineffective to the extent of such
prohibition or unenforceability without invalidating the remaining provisions
hereof in that jurisdiction or affecting the validity or enforceability of such
provision in any other jurisdiction.

             [THE REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]

                                      -3-
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto have executed this Amendment and
Waiver or caused it to be executed by their duly authorized officers, all as of
the day and year first above written.


BORROWERS:               AMERICAN TOWER, L.P. (formerly known as
                         American Tower Systems, L.P.)

                         By ATC GP INC. (formerly known as ATSC GP Inc.),
                         its General Partner


                         By:___________________________________________

                              Title:___________________________________


                         AMERICAN TOWERS, INC. (formerly known as
                         American Tower Systems (Delaware), Inc.)


                         By:___________________________________________

                              Title:___________________________________


ADMINISTRATIVE AGENT
AND BANKS:               TORONTO DOMINION (TEXAS), INC., as
                         Administrative Agent for itself and the Banks and as a
                         Bank


                         By:___________________________________________

                              Title:___________________________________


                                                                Signature Page 1


<PAGE>
 
                         BANK OF AMERICA NATIONAL TRUST AND
                         SAVINGS ASSOCIATION, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         BANK OF MONTREAL, as a Bank


                         By:___________________________________________

                              Title:__________________________________


                         THE BANK OF NEW YORK, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         THE BANK OF NOVA SCOTIA, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         BANK OF SCOTLAND, as a Bank


                         By:__________________________________________

                              Title:__________________________________

                                                                Signature Page 2


<PAGE>
 
                         BANKBOSTON, N.A., as a Bank


                         By:__________________________________________

                              Title:__________________________________


                         BANKERS TRUST COMPANY, as a Bank


                         By:__________________________________________

                              Title:__________________________________


                         BARCLAYS BANK, PLC, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         THE CHASE MANHATTAN BANK, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         THE CIT GROUP/EQUIPMENT FINANCING, INC., as a Bank


                         By:___________________________________________

                              Title:___________________________________

                                                                Signature Page 3


<PAGE>
 
                         CITY NATIONAL BANK, as a Bank


                         By:__________________________________________

                              Title:__________________________________


                         COBANK, ACB, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         CREDIT LYONNAIS NEW YORK BRANCH, as a
                         Bank


                         By:___________________________________________

                              Title:___________________________________


                         CREDIT SUISSE FIRST BOSTON, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         By:___________________________________________

                              Title:___________________________________


                                                                Signature Page 4


<PAGE>
 
                         CRESTAR BANK, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         DRESDNER BANK AG, NEW YORK AND GRAND
                         CAYMAN BRANCHES, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         By:___________________________________________

                              Title:___________________________________


                         FIRST NATIONAL BANK OF MARYLAND, as a
                         Bank


                         By:___________________________________________

                              Title:___________________________________


                         FLEET NATIONAL BANK, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                                                                Signature Page 5


<PAGE>
 
                         GENERAL ELECTRIC CAPITAL CORPORATION, as
                         a Bank


                         By:___________________________________________

                              Title:___________________________________


                         THE HOWARD BANK, N.A., as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         KEY CORPORATE CAPITAL INC., as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         LEHMAN COMMERCIAL PAPER INC., as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         THE LONG-TERM CREDIT BANK OF JAPAN, LTD.,
                         NEW YORK BRANCH, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                                                                Signature Page 6


<PAGE>
 
                         MELLON BANK, N.A., as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         MERCANTILE BANK NATIONAL ASSOCIATION,
                         as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         MORGAN STANLEY SENIOR FUNDING, INC., as
                         a Bank


                         By:___________________________________________

                              Title:___________________________________


                         NATIONAL BANK OF CANADA, as a Bank


                         By:___________________________________________

                              Title:___________________________________

                         By:___________________________________________

                              Title:___________________________________



                                                                Signature Page 7


<PAGE>
 
                         PNC BANK, NATIONAL ASSOCIATION, as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         SOCIETE GENERALE, NEW YORK BRANCH, as a
                         Bank


                         By:___________________________________________

                              Title:___________________________________


                         STATE STREET BANK AND TRUST COMPANY, as
                         a Bank


                         By:___________________________________________

                              Title:___________________________________


                         UNION BANK OF CALIFORNIA, N.A., as a Bank


                         By:___________________________________________

                              Title:___________________________________


                         US TRUST, as a Bank


                         
                         By:___________________________________________

                              Title:___________________________________


                                                                Signature Page 8


<PAGE>
 
                         SYNDICATED LOAN FUNDING TRUST

                         By Lehman Commercial Paper, Inc., not in its individual
                         capacity, but solely as Asset Manager


                         By:___________________________________________

                              Title:___________________________________



                                                                Signature Page 9



<PAGE>

                                                                  EXHIBIT 10.24A
 
                  AMENDMENT TO REGISTRATION RIGHTS AGREEMENT


     This Amendment (the "Amendment") is made and entered into as of February 4,
1999, by and among American Tower Corporation, a Delaware corporation (formerly
American Tower Systems Corporation, "ATC"), and the undersigned Stockholders (as
defined in the Registration Rights Agreement (the "Original Agreement") made and
entered into as of January 22, 1998, by and among ATC and the Stockholders
parties thereto.

     WHEREAS, ATC and the Stockholders parties thereto have heretofore executed
the Original Agreement; and

     WHEREAS, ATC proposes to enter into a registration rights agreement with
Credit Suisse First Boston Corporation ("CSFB") substantially similar in terms
and conditions to the Original Agreement (the "CSFB Agreement"); and

     WHEREAS, pursuant to the terms and condition of the Original Agreement the
Stockholders would have the right, pursuant to the provisions of Section 1(a) of
the Original Agreement,  to require that their Registrable Securities (as
defined in the Original Agreement") be registered pursuant to any registration
statement filed pursuant to the provisions of the CSFB Agreement; and

     WHEREAS, the Stockholders were recently afforded the opportunity to have
their Registrable Securities registered on another registration statement filed
by ATC under the Securities Act of 1933, as amended; and

     WHEREAS, ATC desires that the Original Agreement be amended so that the
rights of registration of the Stockholders will not apply to registration
statements filed pursuant to the provisions of the CSFB Agreement;

     NOW, THEREFORE, in consideration of the recitals, the mutual covenants and
agreements herein contained, and other valuable consideration, the receipt,
adequacy and sufficiency of which are hereby acknowledged, the parties hereto,
intending to be legally bound, do hereby covenant and agree as follows:
 
     Section 1.  Amendment of Section 1(a) of the Original Agreement.  A new
                 ---------------------------------------------------        
paragraph shall be added at the end of Section 1(a) of the Original Agreement
reading in its entirety as follows:

          Anything in this Section 1(a) to the contrary notwithstanding, the
     provisions of this Section 1(a) shall not apply to any registration
     statement filed by ATC under the Securities Act pursuant to the provisions
     of the CSFB Agreement.

     Section 2.  Other Amendments.  Section 11 of the Original Agreement shall
                 ----------------                                             
be amended to add the following definition in its appropriate alphabetical
order:

          "CSFB AGREEMENT" shall mean the registration rights agreement, dated
     as of February 4, 1999, by and between ATC and Credit Suisse First Boston,
     as from time to time amended in accordance with its terms.

     Section 3.  Restatement of Original Agreement.  The Original Agreement is
                 ---------------------------------                            
hereby amended and restated in its entirety to (a) reflect the amendments set
forth in this Amendment and (b) reflect the name 
<PAGE>
 
change of American Tower Systems Corporation to American Tower Corporation and
to change all references to the defined term "ATS" to "ATC".

     Section 4.  Miscellaneous.
                 ------------- 

          (a)  Except as modified hereby the provisions of the Original
     Agreement shall remain in fully force and effect.

          (b)  The provisions of Section 12 are incorporated herein with the
     same force and effect as though set forth herein in their entirety.

     IN WITNESS WHEREOF, the parties hereto have executed or caused this
Agreement to be executed as of February 4, 1999.

                    American Tower Corporation


                    By:__________________________________
                       Name:  Steven B. Dodge
                       Title: Chairman of the Board and Chief Executive Officer

                    ______________________________________
                    Steven B. Dodge

                    Chase Equity Associates, L.P.
                    By Chase Capital Partners, General Partner


                    By:______________________________________
                       Name:
                       Title:

                    _________________________________________
                               J. Michael Gearon, Jr.
 
                    The 1997 Gearon Family Trust


                    _________________________________________
                    By: J.  Michael Gearon, Sr., Trustee


                    Chase Manhattan Capital Corporation

                    By:______________________________________
                       Name:
                       Title:

                    _________________________________________
                    Norman A. Bikales                     

                    _________________________________________
                    Joseph L. Winn

                    _________________________________________
                    Alan L. Box

<PAGE>

                                                                   EXHIBIT 10.25


                           AMERICAN TOWER CORPORATION


                            STOCK PURCHASE AGREEMENT


     THIS AGREEMENT (this "Agreement")  is made as of February 4, 1999 by and
between American Tower Corporation, a Delaware corporation ("ATC") and Credit
Suisse First Boston Corporation ("CSFB").

     WHEREAS, CSFB desires to purchase, and ATC is willing to sell 500,000
shares of Class A Common Stock, par value $.01 per share,  of ATC (the "ATC
Common Stock"), all on the terms and subject to the conditions of this
Agreement;  and

     WHEREAS, ATC and CSFB wish to provide for the terms and conditions of the
purchase and sale of the ATC Common Stock and certain related matters;

     NOW, THEREFORE, for and in consideration of the mutual covenants and
agreements herein contained, and other valuable consideration, the receipt and
adequacy whereof are hereby acknowledged, the parties hereto, intending to be
legally bound, do hereby covenant and agree as follows:
<PAGE>
 
     SECTION 1.  Authorization and Closing.
                 ------------------------- 

     (a)  Authorization of the Securities.  ATC has authorized the issue and 
          ------------------------------- 
sale to CSFB, and CSFB has agreed to purchase 500,000 shares of ATC Common Stock
(collectively, the "Subject Shares") at a purchase price equal to $26.3125 per
share.

     (b)  Purchase and Sale of the Subject Shares.  At the Closing, subject to
          ---------------------------------------                             
the terms and conditions set forth herein, ATC shall issue to CSFB, and CSFB
shall acquire from ATC, the Subject Shares set forth in Section 1(a).  CSFB will
make payment in the form of a wire transfer of immediately available funds or
bank cashier's or certified check; and

     (c)  The Closing.  The closing (the "Closing") of the issue and acquisition
          -----------                                                           
of the Subject Shares shall take place at the offices of Sullivan & Cromwell,
New York, New York on February 9, 1999 at 10:00 a.m. or at such other date and
place as may be mutually agreeable to ATC and CSFB.  Any wire transfer of funds
shall be made to such bank account in the United States as shall have been
designated in writing by ATC a reasonable time prior to the Closing.

     SECTION 2.  Conditions of CSFB's Obligation.  The obligation of CSFB to
                 -------------------------------                            
acquire the Subject Shares to be purchased by CSFB at the Closing is subject to
the satisfaction (or waiver in writing by CSFB) as of the Closing of the
following conditions:

          (a)    Representations and Warranties; Covenants.  The 
                 -----------------------------------------
     representations and warranties contained in Section 4 shall be true and
     correct in all material respects at and as of the Closing as though then
     made, except to the extent of changes caused by the transactions expressly
     contemplated herein, and ATC shall have performed in all material respects
     all of the covenants required to be performed by it hereunder prior to the
     Closing.

          (b)    Registration Rights Agreement.  ATC shall have executed and 
                 -----------------------------
     delivered to CSFB an agreement substantially in the form of Exhibit A
     attached hereto and made a part hereof (the "Registration Rights
     Agreement").

          (c)    Legal Opinion.  ATC shall have delivered to CSFB an opinion of
                 -------------                                                 
     Sullivan & Worcester LLP, counsel for ATC, as to the due organizations and
     corporate existence of ATC, the due authorization, execution, delivery and
     binding effect of this Agreement, and the due authorization and valid
     issuance of the Subject Shares and their fully paid and nonassessable
     status.
          (d)    Proceedings.  All corporate and other proceedings taken or
                 -----------                                               
     required to be taken by ATC in connection with the transactions
     contemplated hereby to be consummated at or prior to the Closing and all
     documents incident thereto shall be reasonably satisfactory in form and
     substance to CSFB and their counsel.

          (e)    Closing Documents.  ATC shall have delivered to CSFB such 
                 -----------------
     documents, instruments and certificates relating to the transactions
     contemplated by this Agreement as CSFB or its counsel may reasonably
     request.

                                      -2-
<PAGE>
 
     SECTION 3.  Conditions of ATC's Obligation.  The obligation of ATC to issue
                 ------------------------------                                 
and sell the Subject Shares to CSFB at the Closing is subject to the
satisfaction (or waiver in writing by ATC) as of the Closing of the following
conditions:

          (a)    Representations and Warranties; Covenants.  The representations
                 -----------------------------------------                      
     and warranties of CSFB contained in Section 5 shall be true and correct in
     all material respects at and as of the Closing as though then made, except
     to the extent of changes caused by the transactions expressly contemplated
     herein, and CSFB shall have performed in all material respects all of the
     covenants required to be performed by him or it hereunder prior to the
     Closing.

          (b)    Proceedings.  All corporate and other proceedings taken or
                 -----------                                               
     required to be taken by CSFB in connection with the transactions
     contemplated hereby to be consummated at or prior to the Closing and all
     documents incident thereto shall be reasonably satisfactory in form and
     substance to ATC and its counsel.

          (c)    Closing Documents.  CSFB shall have delivered to ATC such
                 -----------------                                        
     documents, instruments and certificates relating to the transactions
     contemplated by this Agreement as ATC or its counsel may reasonably
     request.

          SECTION 4.  Representations and Warranties of ATC.  As a material
                      -------------------------------------                
inducement to CSFB to enter into this Agreement and purchase the Subject Shares,
ATC hereby makes the following representations and warranties to CSFB.

          (a)    Organization and Corporate Power.  ATC and each of its 
                 --------------------------------
     subsidiaries is an entity duly organized, validly existing and in good
     standing under the laws of its jurisdiction of organization and is
     qualified to do business in each jurisdiction in which the failure so to
     qualify would reasonably be expected to have a material adverse effect on
     the business, financial condition or results of operation of ATC and its
     subsidiaries taken as a whole. ATC and each of its subsidiaries has all
     requisite corporate and other power and authority and all material
     licenses, permits and authorizations necessary to own and operate its
     properties, to carry on its businesses as now conducted and presently
     proposed to be conducted and, in the case of ATC, to execute and deliver
     and consummate the transactions contemplated by this Agreement.

          (b)    Material Statements and Omissions; Absence of Events.  The
                 ----------------------------------------------------      
     information with respect to ATC included in the prospectus included in the
     registration statement to be filed on or about February 8, 1999 heretofore
     delivered by ATC to CSFB (the "ATC Prospectus") does not and will not
     contain any untrue statement of a material fact and does not and will not
     omit to state any material fact required to make any statement contained
     herein or therein, in light of the circumstances under which they were
     made, not misleading.  Since the date of the most recent financial
     statements set forth in the ATC Prospectus, except to the extent described
     in the ATC Prospectus, there has been no material adverse change in ATC.

          (c)    Capital Stock and Related Matters.
                 --------------------------------- 

          (i)    The authorized and issued capital stock of ATC is as set forth
     in the ATC Prospectus. All of the outstanding shares of the ATC Common
     Stock are, and all of the Subject 

                                      -3-
<PAGE>
 
     Shares will, upon issuance pursuant to the provisions of this Agreement,
     be, duly authorized, validly issued, fully paid and nonassessable.

          (ii)   There are no statutory or, to the best of ATC's knowledge,
     contractual stockholder preemptive rights or rights of refusal with respect
     to the issuance of the Subject Shares.  ATC has not violated any applicable
     federal or state securities laws in connection with the offer, sale or
     issuance of any of its capital stock, and, based on the representations and
     warranties of CSFB set forth in Section 5(g), the offer, sale and issuance
     of the Subject Shares hereunder does not require registration under the
     Securities Act of 1933, as amended (the "Securities Act") or any applicable
     state securities laws.

          (d)    Authorization; No Breach.  The execution, delivery and 
                 ------------------------
     performance of this Agreement, the Registration Rights Agreement and all
     other agreements contemplated hereby to which ATC is or will be a party
     have been duly authorized by ATC. This Agreement, the Registration Rights
     Agreement and all other agreements contemplated hereby each constitutes a
     valid and binding obligation of ATC, enforceable in accordance with its
     terms, except as (i) the enforceability thereof may be limited by
     bankruptcy, reorganization, insolvency, moratorium, fraudulent conveyance
     or other laws of general applicability affecting the enforcement of
     creditors' or secured parties' rights or debtors' obligations generally and
     (ii) the availability of specific performance or other equitable remedies
     may be limited by equitable principles of general applicability (whether
     such matter is considered in a proceeding at law or in equity). The (x)
     execution and delivery by ATC of this Agreement and the Registration Rights
     Agreement, (y) offering, sale and issuance of the Subject Shares hereunder,
     and (z) fulfillment of and compliance with the respective terms hereof and
     thereof by ATC, do not and shall not, except in respect of clause (z)
     above, for filings and other actions to be performed upon the occurrence of
     certain future events, as contemplated by the Registration Rights
     Agreement, (i) conflict with or result in a breach of the terms, conditions
     or provisions of, (ii) constitute a default under, (iii) result in the
     creation of any lien, security interest, charge or encumbrance upon ATC's
     or any subsidiary's capital stock or assets pursuant to, (iv) give any
     third party the right to modify, terminate or accelerate any obligation
     under, (v) result in a violation of, or (vi) require any authorization,
     consent, approval, exemption or other action by or notice to any court or
     administrative or governmental body pursuant to, the charter or by-laws or
     other organizational documents of ATC or any subsidiary, or any law,
     statute, rule or regulation to which ATC or any subsidiary is subject, or
     any contract, agreement, instrument, order, judgment or decree to which ATC
     or any subsidiary is subject, which such authorization, consent, approval,
     exemption, action or notice has not been obtained, except in all cases for
     such exceptions as would not, individually or in the aggregate, have a
     material adverse effect on the business, financial conditions or results of
     operation of ATC and its subsidiaries taken as a whole.

          (e)    Brokerage.  There are no claims for brokerage commissions, 
                 ---------
     finders' fees or similar compensation in connection with the transactions
     contemplated by this Agreement based on any arrangement or agreement
     binding upon ATC or any subsidiary. ATC shall pay, and hold CSFB harmless
     against, any liability, loss or expense (including, without limitation,
     reasonable attorneys' fees and out-of-pocket expenses) arising in
     connection with any such claim.

          (f)    Governmental Consent, etc.  No permit, consent, approval or
                 -------------------------                                  
authorization of, or declaration to or filing with, any governmental authority
is required in connection with the execution, delivery and performance by ATC of
this Agreement or the other agreements 

                                      -4-
<PAGE>
 
contemplated hereby, or the consummation by ATC of any other transactions
contemplated hereby or thereby, except as expressly contemplated herein or
therein or in the exhibits hereto or thereto.

     SECTION 5.  Representations and Warranties of each CSFB.  As a material
                 -------------------------------------------                
inducement to ATC to enter into this Agreement and issue and sell the Subject
Shares CSFB hereby makes the following representations and warranties to ATC.

          (a)    Organization and Power.  CSFB is duly organized, validly 
                 ----------------------
     existing and in good standing under the laws of its jurisdiction of
     organization and has all requisite corporate or other power and authority
     to execute and deliver and consummate the transactions contemplated by this
     Agreement.

          (b)    Authorization; No Breach.  The execution, delivery and 
                 ------------------------
performance of this Agreement, the Registration Rights Agreement and all other
agreements contemplated hereby to which CSFB is or will be a party have been
duly authorized by CSFB. This Agreement, the Registration Rights Agreement and
all other agreements contemplated hereby to which CSFB is or will be a party
each constitutes a valid and binding obligation of such CSFB, enforceable in
accordance with its terms, except as (i) the enforceability thereof may be
limited by bankruptcy, reorganization, insolvency, moratorium, fraudulent
conveyance or other laws of general applicability affecting the enforcement of
creditors' or secured parties' rights or debtors' obligations generally and (ii)
the availability of specific performance or other equitable remedies may be
limited by equitable principles of general applicability (whether such matter is
considered in a proceeding at law or in equity). The (x) execution and delivery
by CSFB of this Agreement, the Registration Rights Agreement and all other
agreements contemplated hereby to which CSFB is or will be a party, and (y)
fulfillment of and compliance with the respective terms hereof and thereof by
CSFB, do not and shall not, except in respect of clause (y) above, for filings
and other actions to be performed upon the occurrence of certain future events,
as contemplated by the Registration Rights Agreement, (i) conflict with or
result in a breach of the terms, conditions or provisions of, (ii) constitute a
default under, (iii) result in the creation of any lien, security interest,
charge or encumbrance upon CSFB's assets pursuant to, (iv) give any third party
the right to modify, terminate or accelerate any obligation under, (v) result in
a violation of, or (vi) require any authorization, consent, approval, exemption
or other action by or notice to any court or administrative or governmental body
pursuant to, any law, statute, rule or regulation to which CSFB is subject, or
any contract, agreement, instrument, order, judgment or decree to which CSFB is
subject, which such authorization, consent, approval, exemption, action or
notice has not been obtained.

          (c)    Brokerage.  There are no claims for brokerage commissions, 
                 --------- 
     finders' fees or similar compensation in connection with the transactions
     contemplated by this Agreement based on any arrangement or agreement
     binding upon CSFB. CSFB shall pay and hold ATC harmless against any
     liability, loss or expense (including, without limitation, reasonable
     attorneys' fees and out-of-pocket expenses) arising in connection with any
     such claim.

          (d)    Governmental Consent, etc.  No permit, consent, approval or
                 -------------------------                                  
     authorization of, or declaration to or filing with, any governmental
     authority is required in connection with the execution, delivery and
     performance by CSFB of this Agreement or the other agreements contemplated
     hereby, or the consummation by CSFB of any other transactions contemplated
     hereby 

                                      -5-
<PAGE>
 
     or thereby, except as expressly contemplated herein or therein, in the
     exhibits hereto or thereto or in the ATC Prospectus.

          (e)    Investment Representation.  CSFB is an "accredited investor" 
                 -------------------------
     as such term is defined in Rule 501 of Regulation D ("Regulation D")
     promulgated under the Securities Act. The Subject Shares to be acquired by
     CSFB are being acquired solely for the account of CSFB for purposes of
     investment and not with a view to the sale, transfer or other distribution
     thereof, as those terms are used in the Securities Act and the rules and
     regulations promulgated thereunder; provided, however, that nothing
     contained herein shall prevent CSFB and subsequent holders of Subject
     Shares from transferring such securities in compliance with the applicable
     provisions of the Securities Act (including without limitation Rule 144
     promulgated thereunder or pursuant to registration) and applicable state
     securities laws. CSFB covenants and agrees that it will not sell, assign,
     transfer or otherwise dispose of any of the Subject Shares to be acquired
     by CSFB in violation of the Securities Act or applicable state securities
     laws. Each certificate for the Subject Shares shall be imprinted with a
     legend in substantially the following form:

          "THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
          REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED.  THE TRANSFER
          OF THE SECURITIES REPRESENTED BY THIS CERTIFICATE IS SUBJECT TO THE
          CONDITIONS SPECIFIED IN THE STOCK PURCHASE AGREEMENT, DATED AS OF
          FEBRUARY 4, 1999, BETWEEN THE ISSUER ("ATC") AND CREDIT SUISSE FIRST
          BOSTON CORPORATION, AND ATC RESERVES THE RIGHT TO REFUSE THE TRANSFER
          OF SUCH SECURITIES UNTIL SUCH CONDITIONS HAVE BEEN FULFILLED WITH
          RESPECT TO SUCH TRANSFER.  A COPY OF SUCH CONDITIONS SHALL BE
          FURNISHED BY ATC TO THE HOLDER HEREOF UPON WRITTEN REQUEST AND WITHOUT
          CHARGE."  THIS LEGEND SHALL EXPIRE AUTOMATICALLY, AND SHALL BE
          REMOVED, ON FEBRUARY 5, 2001.

     SECTION 6.  Miscellaneous.
                 ------------- 

     (a)  Termination.  This Agreement may be terminated with the mutual consent
          -----------                                                           
of CSFB and ATC.

     (b)  Amendment.  This Agreement may be amended, from time to time, by the
          ---------                                                           
parties hereto at any time prior to the Closing Date but only by an instrument
in writing signed by the parties hereto or their respective successors or
assigns.

     (c)  Waiver. At any time prior to the Closing Date, the parties may, either
          ------                                                                
generally or in a particular instance and either retrospectively or
prospectively, extend the time for the performance of any of the obligations or
other acts of the other, and waive compliance by the other with any of the
agreements, covenants, conditions or other provisions contained herein.  Any
such extension or waiver shall be valid only if set forth in an instrument in
writing signed by the party or parties to be bound thereby.

     (d)  Expenses. CSFB agrees to pay the reasonable out-of-pocket expenses of
          --------                                                             
ATC incident to the negotiation, preparation, performance and enforcement of
this Agreement (including all reasonable fees 

                                      -6-
<PAGE>
 
and expenses of ATC's counsel, accountants and other consultants, advisors and
representatives for all activities of such persons undertaken pursuant to this
Agreement).

     (e)  Notices.  All notices and other communications which by any provision
          -------                                                              
of this Agreement are required or permitted to be given shall be given in
writing and shall be (a) mailed by first-class or express mail, postage prepaid,
or by recognized courier service, (b) sent by telecopy or other form of rapid
transmission, confirmed by mailing (by first class or express mail, postage
prepaid, or by recognized courier service) written confirmation at substantially
the same time as such rapid transmission, or (c) personally delivered to the
receiving party (which if, other than an individual, shall be an officer or
other responsible party of the receiving party).  All such notices and
communications shall be mailed, sent or delivered as follows:

     If to ATC:          116 Huntington Avenue
                         Boston, Massachusetts 02116
                         Attention:   Joseph L. Winn, Chief Financial Officer
                         Telecopier No.:  (617) 375-7575

     with a copy to:     Sullivan & Worcester LLP
                         One Post Office Square
                         Boston, Massachusetts 02109
                         Attention:  Norman A. Bikales, Esq.
                         Telecopier No.:  (617) 338-2880; and

     If to CSFB:         11 Madison Avenue
                         New York, New York  10010
                         Attention:  Kristin M. Allen, Managing Director
                         Telephone No.:  (212) 325-2459

     with a copy to:     Sullivan & Cromwell
                         125 Broad Street
                         New York, New York 10004
                         Attention: John T. Bostelman, Esq.
                         Telecopier No.: (212) 558-3588

or to such other person(s), telex or facsimile number(s) or address(es) as the
party to receive any such communication or notice may have designated by written
notice to the other party.

     (f)  Specific Performance; Other Rights and Remedies.  Each party 
          -----------------------------------------------
recognizes and agrees that in the event the other party should refuse to perform
any of its obligations under this Agreement, the remedy at law would be
inadequate and agrees that for breach of such provisions, each party shall, in
addition to such other remedies as may be available to it at law or in equity,
be entitled to injunctive relief and to enforce its rights by an action for
specific performance to the extent permitted by applicable law. Each party
hereby waives any requirement for security or the posting of any bond or other
surety in connection with any temporary or permanent award of injunctive,
mandatory or other equitable relief. Nothing herein contained shall be construed
as prohibiting each party from pursuing any other remedies available to it under
applicable law or pursuant to the provisions of this Agreement for such breach
or threatened breach, including without limitation the recovery of damages.

                                      -7-
<PAGE>
 
     (g)  Survival of Representations, Warranties, Covenants and Agreements.  
          -----------------------------------------------------------------
All of the representations, warranties, covenants and agreements set forth in
this Agreement shall survive the Closing.

     (h)  Severability.  If any term or provision of this Agreement shall be 
          ------------
held or deemed to be, or shall in fact be, invalid, inoperative, illegal or
unenforceable as applied to any particular case in any jurisdiction or
jurisdictions, or in all jurisdictions or in all cases, because of the
conflicting of any provision with any constitution or statute or rule of public
policy or for any other reason, such circumstance shall not have the effect of
rendering the provision or provisions in question invalid, inoperative, illegal
or unenforceable in any other jurisdiction or in any other case or circumstance
or of rendering any other provision or provisions herein contained invalid,
inoperative, illegal or unenforceable to the extent that such other provisions
are not themselves actually in conflict with such constitution, statute or rule
of public policy, but this Agreement shall be reformed and construed in any such
jurisdiction or case as if such invalid, inoperative, illegal or unenforceable
provision had never been contained herein and such provision reformed so that it
would be valid, operative and enforceable to the maximum extent permitted in
such jurisdiction or in such case. Notwithstanding the foregoing, in the event
of any such determination the effect of which is to affect materially and
adversely either CSFB or ATC, the parties shall negotiate in good faith to
modify this Agreement so as to effect the original intent of the parties as
closely as possible to the fullest extent permitted by applicable law in an
acceptable manner to the end that the intent and purpose of this Agreement is
fulfilled and consummated to the maximum extent possible.

     (i)  Counterparts.  This Agreement may be executed in several counterparts,
          ------------                                                          
each of which shall be deemed an original, but all of which together shall
constitute one and the same instrument, binding upon all of the parties.  In
pleading or proving any provision of this Agreement, it shall not be necessary
to produce more than one of such counterparts.

     (j)  Section Headings.  The headings contained in this Agreement are for
          ----------------                                                   
reference purposes only and shall not in any way affect the meaning or
interpretation of this Agreement.

     (k)  Governing Law.  The validity, interpretation, construction and
          -------------                                                 
performance of this Agreement shall be governed by, and construed in accordance
with, the applicable laws of the United States of America and the laws of the
Commonwealth of Massachusetts applicable to contracts made and performed in such
State and, in any event, without giving effect to any choice or conflict of laws
provision or rule that would cause the application of domestic substantive laws
of any other jurisdiction, except to the extent the corporate laws of the State
of Delaware are applicable.  Anything in this Agreement to the contrary
notwithstanding, in the event of any dispute between the parties which results
in a legal action, litigation or other proceeding, the prevailing party shall be
entitled to receive from the non-prevailing party reimbursement for reasonable
legal fees and expenses incurred by such prevailing party in such legal action,
litigation or other proceeding.

     (l)  Further Acts.  Each party agrees that at any time, and from time to
          ------------                                                       
time, before and after the consummation of the transactions contemplated by this
Agreement, it will do all such things and execute and deliver all such other
agreements, instruments or other documents and other assurances, as the other
party or its counsel reasonably deems necessary or desirable in order to carry
out the terms and conditions of this Agreement and the transactions contemplated
hereby or to facilitate the enjoyment of any of the rights created hereby or to
be created hereunder.

                                      -8-
<PAGE>
 
     (m)  Assignment.  This Agreement shall not be assignable by any party and
          ----------                                                          
any such assignment shall be null and void, except that it shall inure to the
benefit of and be binding upon any successor to each party by operation of law,
including by way of merger, consolidation or sale of all or substantially all of
its assets, and, in the case of CSFB, any person to whom CSFB shall have
transferred any of the Subject Shares not in violation of this Agreement.
Notwithstanding the foregoing, ATC may assign its rights and remedies hereunder
to any bank or other financial institution which has loaned funds or otherwise
extended credit to it; and CSFB may assign its rights and obligations under this
Agreement and the Registration Rights Agreement, and may assign the Subject
Shares, to an affiliate that agrees to be bound thereby.

     (n)  Parties in Interest.  This Agreement shall be binding upon and inure
          -------------------                                                 
solely to the benefit of the parties and their permitted successors and assigns,
and nothing in this Agreement, express or implied, is intended to or shall
confer upon any person any right, benefit or remedy of any nature whatsoever
under or by reason of this Agreement, except as otherwise provided in Section
6(m).

     (o)  Certain Definitions.  Terms defined in the singular shall have a
          -------------------                                             
comparable meaning when used in the plural, and vice versa, and the reference to
any gender shall be deemed to include all genders. References to "hereof,"
"herein" or similar terms are intended to refer to the Agreement as a whole and
not a particular section, and references to "this Section" are intended to refer
to the entire section or article and not a particular subsection thereof.

     (p)  Mutual Drafting.  This Agreement is the result of the joint efforts of
          ---------------                                                       
ATC and CSFB, and each provision hereof has been subject to the mutual
consultation, negotiation and agreement of the parties and there shall be no
construction against either party based on any presumption of that party's
involvement in the drafting thereof.  Each of the parties is a sophisticated
legal entity or individual that was advised by experienced counsel and, to the
extent it deemed necessary, other advisors in connection with this Agreement.


                    SIGNATURES APPEAR ON THE FOLLOWING PAGE

                                      -9-
<PAGE>
 
     IN WITNESS WHEREOF, each of the parties hereto had caused this Agreement to
be duly executed and delivered as of the day and year first above written.


                              American Tower Corporation
 
 
                              By: ________________________________
                              Name:  Steven B. Dodge
                              Title: President and Chief Executive Officer



                              Credit Suisse First Boston Corporation



                              By: ________________________________
                              Name:
                              Title:

                                      -10-

<PAGE>

                                                                   EXHIBIT 10.26


                                                                       EXHIBIT A



                         REGISTRATION RIGHTS AGREEMENT

                                    between

                          AMERICAN TOWER CORPORATION

                                      and

                    CREDIT SUISSE FIRST BOSTON CORPORATION



                               February 4, 1999
<PAGE>
 
                               TABLE OF CONTENTS

<TABLE> 
<S>                                                                                                              <C> 
1.       Registration of Securities............................................................................   1
         ---------------------------
         (a)      Registration by ATC..........................................................................   1
                  -------------------
         (b)      Registration at CSFB's Request...............................................................   2
                  ------------------------------
         (c)      Registration Generally.......................................................................   3
                  ----------------------
         [(d)     Restrictions on Registration.................................................................   7
                  ----------------------------
         (e)      Additional Restrictions on Registration......................................................   8
                  ---------------------------------------

2.       Conditions to Registration............................................................................   8
         --------------------------

3.       Indemnification.......................................................................................   9
         ---------------
         (a)      Indemnification by ATC.......................................................................   9
                  ----------------------
         (b)      Indemnification by CSFB......................................................................   9
                  -----------------------
         (c)      Procedure....................................................................................  10
                  ---------
         (d)      Contribution.................................................................................  11
                  ------------

4.       Exchange Act Registration.............................................................................  12
         -------------------------

5.       Termination of Registration Obligations...............................................................  13
         ---------------------------------------

6.       Registration Rights of Others.........................................................................  14
         -----------------------------

7.       Mergers, etc..........................................................................................  14
         ------------

8.       Annual and Quarterly Reports; Other Information.......................................................  14
         -----------------------------------------------

9.       Withdrawals...........................................................................................  14
         -----------

10.      Definitions...........................................................................................  14
         -----------

11.      Miscellaneous.........................................................................................  18
         -------------
         (a)      Assignment; Successors and Assigns...........................................................  18
                  ----------------------------------
         (b)      Specific Performance; Other Rights and Remedies..............................................  18
                  -----------------------------------------------
         (c)      Expenses.....................................................................................  18
                  --------
         (d)      Entire Agreement.............................................................................  19
                  ----------------
         (e)      Waivers; Amendments..........................................................................  19
                  -------------------
         (f)      Notices......................................................................................  19
                  -------
         (g)      Severability.................................................................................. 19
                  ------------
         (h)      Counterparts.................................................................................. 20
                  ------------
         (i)      Section Headings.............................................................................  20
                  ----------------
         (j)      Governing Law................................................................................  20
                  -------------
         (k)      Further Acts.................................................................................  20
                  ------------
</TABLE> 
<PAGE>
 
                         REGISTRATION RIGHTS AGREEMENT


     This Registration Rights Agreement (this "Agreement") is made and entered
into as of February 4, 1999, by and between American Tower Corporation, a
Delaware corporation ("ATC"), and Credit Suisse First Boston Corporation
("CSFB"), which has heretofore agreed to acquire or has acquired the Registrable
Securities.

     WHEREAS, ATC has entered into a Stock Purchase Agreement, dated as of
February 4, 1999, with CSFB (the "ATC Stock Purchase Agreement") relating to the
issue and sale of shares of Class A Common Stock of ATC (the "ATC Common
Stock"), and it is a condition of the obligation of CSFB to consummate the
transactions contemplated by the ATC Stock Purchase Agreement that ATC enter
into an agreement substantially in the form of this Agreement; and

     WHEREAS, ATC has entered into and may in the future enter into agreements
pursuant to which it has agreed or will have agreed to issue securities the
holders of which have required or will require registration rights of a nature
set forth in this Agreement;

     NOW, THEREFORE, in consideration of the recitals, the mutual covenants and
agreements herein contained, and other valuable consideration, the receipt,
adequacy and sufficiency of which are hereby acknowledged, the parties hereto,
intending to be legally bound, do hereby covenant and agree as follows:

     1.   Registration of Securities.
          --------------------------

     (a)  Registration by ATC.  If at any time or from time to time ATC shall 
          -------------------
propose to file on its behalf or on behalf of any of its security holders a
registration statement under the Securities Act with respect to any class of
Common Stock, except in connection with an Excluded Offering, ATC shall, except
to the extent not required to do so pursuant to the provisions of Section 1(d)
or 1(e), in each case:

          (i)    promptly give written notice to each Stockholder at least
     thirty (30) days (or such shorter period as ATC deems reasonable under the
     circumstances) before the anticipated filing date. Such notice shall
     include the anticipated offering price or range thereof and the plan of
     distribution;

          (ii)   include in such registration (and any related qualification
     under blue sky or other state securities laws), and, at the request of a
     Stockholder, in any underwriting involved therein, all Registrable
     Securities specified in a written request or requests, made within ten (10)
     business days after such written notice from ATC, by any Stockholder; and

          (iii)  use its reasonable business efforts to cause the managing
     underwriter or underwriters of any proposed underwritten offering of ATC
     Common Stock to permit the Registrable Securities requested to be included
     in the Registration Statement for such offering on the same terms and
     conditions as the ATC Common Stock included therein. Notwithstanding the
     foregoing, if the managing underwriters of such offering deliver a written
     opinion to the holders of such Registrable Securities that marketing
     considerations require a limitation on the Registrable Securities included
     in any Registration Statement filed under this Section, then, subject to
     the advice of said managing

                                      -1-
<PAGE>
 
     underwriter or underwriters as to the size and composition of the offering,
     and subject to the provisions of Section 1(d), such limitation will be
     imposed pro rata (based upon the relative proposed public offering price of
     the Registrable Securities proposed to be included) among all holders of
     Registrable Securities who requested inclusion in the registration pursuant
     to this Section. For purposes of this clause (iii), "Registrable
     Securities" shall include Registrable Securities as defined under the 1998
     Registration Rights Agreement (as defined below).

     If any Stockholder desires to have Registrable Securities registered under
this Section, it shall be required so to advise ATC in writing within ten (10)
business days after the date of ATC's notice, setting forth the number or amount
of Registrable Securities for which registration is so requested. Neither the
delivery of the notice by ATC nor of the request by any Stockholders shall in
any way obligate ATC to file a Registration Statement and, notwithstanding such
filing, ATC may, at any time prior to the effective date thereof, determine not
to offer the securities to which the registration statement relates without
liability to any of the Stockholders. No registration of Registrable Securities
effected under this Section shall relieve ATC of its obligation to effect
registration of Registrable Securities upon any request made pursuant to the
provisions of Section 1(b).

     Notwithstanding the foregoing, CSFB's rights under this Section 1(a) are
subject to the written consent of the selling stockholders under Section 1(b) of
the Registration Rights Agreement, dated as of January 22, 1998, among ATC and
the stockholders named therein (the "1998 Registration Rights Agreement") but
only to the extent a registration statement is prepared upon the request of such
selling stockholders.

     (b)   Registration at Stockholders' Request.  Upon the written request of 
           -------------------------------------
any Significant Stockholder requesting that ATC effect the registration under
the Securities Act of all or part of the Registrable Securities held by CSFB,
specifying the intended method or methods of disposition of such Registrable
Securities (which may include sales from time to time on a delayed basis), ATC
shall, except to the extent not required to do so pursuant to the provisions of
this Section 1(b) or Section 1(d) or (e), promptly (and in any event within five
(5) business days) give written notice of such requested registration to all
holders of Registrable Securities and thereupon will expeditiously prepare and,
within forty-five (45) days, use its reasonable business efforts to file under
the Securities Act a registration statement and effect the registration of:

           (i)    the Registrable Securities which ATC has been so requested to
     register by CSFB, for disposition in accordance with the intended method of
     disposition stated in such request, and

           (ii)   all other Registrable Securities which ATC has been requested
     to register by the holders of Registrable Securities by written request
     delivered to ATC within ten (10) business days after the giving of such
     notice by ATC (which request shall specify the intended method of
     disposition of such Registrable Securities).

     Each registration requested pursuant to this Section shall be effected by
the filing of a Registration Statement on Form S-1 (or such other form as the
Commission may from time to time require or permit in order to effectuate a
public offering of common stock of a company such as ATC and in a method of
disposition such as that proposed), unless the use of a different form has been
agreed upon in writing by

                                      -2-
<PAGE>
 
holders of not less than a majority in value (based upon the proposed public
offering price) of the Registrable Securities as to which registration has so
been requested. Notwithstanding the preceding sentence, ATC need not so cause a
Registration Statement so filed pursuant to the provisions of this Section on a
Form S-1 (or any successor form) to become effective under the Securities Act on
more than three (3) occasions; provided, however, that there shall be no limit
on the number of times ATC is obligated to file Registration Statements on Form
S-2 or S-3 (or any successor forms) pursuant to the provisions of this Section
(except as contemplated by the definition of Significant Stockholder); and
provided further, however, that any registration of Registrable Securities
requested by one or more Stockholders pursuant to this Section which shall not
have become and remained effective in accordance with the provisions of Section
1(c) shall not be deemed to be a registration for purposes of this Section.

     ATC shall not grant to any person the right to request ATC to register, nor
shall ATC include in any registration pursuant to this Section, any securities
other than the Registrable Securities or securities that are the subject of
registration rights granted prior to the date hereof, without the written
consent of holders of not less than a majority in value (based upon the proposed
public offering price) of the Registrable Securities as to which registration
has been so requested.

     Whenever registration requested by one or more Stockholders pursuant to
this Section is for an underwritten offering, only Registrable Securities which
are to be distributed by the underwriters designated by such Stockholders may be
included in such registration, without the written consent of holders of not
less than a majority in value (based upon the proposed public offering price) of
the Registrable Securities as to which registration has been so requested. If
Stockholders holding not less than a majority in value of the Registrable
Securities (based upon the proposed public offering price) to be included in
such registration shall determine that the number of Registrable Securities
should be limited due to market conditions or otherwise, all holders of
Registrable Securities proposing to sell Registrable Securities in such
underwritten offering shall share pro rata in the number of Registrable
Securities to be excluded from such underwritten offering, such sharing to be
based on the value (based upon the proposed public offering price) of the
respective numbers of Registrable Securities as to which registration has been
requested by such Stockholders.

           (c)   Registration Generally.  If and when ATC shall be required by 
                 ----------------------
the provisions of this Section to effect the registration of Registrable
Securities under the Securities Act, ATC will use its reasonable business
efforts to effect such registration to permit the sale of such Registrable
Securities in accordance with the intended method or methods of disposition
thereof, and pursuant thereto it will, subject to the provisions of Section 1(d)
and 1(e), as expeditiously as possible:

           (i)    before filing a Registration Statement or Prospectus or any
     amendments or supplements thereto, furnish to the holders of the
     Registrable Securities covered by such Registration Statement and the
     managing underwriters, if any, copies of all such documents proposed to be
     filed, which documents will be made available, on a timely basis, for
     review by such holders and underwriters, and their respective counsel, and
     ATC will not file any Registration Statement or amendment thereto or any
     Prospectus or any supplement thereto to which the holders of not less than
     a majority in value (based upon the proposed public offering price) of the
     Registrable Securities covered by such Registration Statement or the
     managing underwriters, if any, shall reasonably have objected;

                                      -3-
<PAGE>
 
           (ii)   prepare and file with the Commission such amendments and post-
     effective amendments to any Registration Statement, and such supplements to
     the Prospectus, as may be reasonably requested by any holder of Registrable
     Securities included in such Registration Statement or any underwriter of
     Registrable Securities or as may be required by the rules, regulations or
     instructions applicable to the registration form utilized by ATC or by the
     Securities Act, the Exchange Act or otherwise necessary to keep such
     Registration Statement effective for the applicable period and cause the
     Prospectus as so supplemented to be filed pursuant to Rule 424 under the
     Securities Act; and comply with the provisions of the Securities Act with
     respect to the disposition of all Registrable Securities covered by such
     Registration Statement during the applicable period in accordance with the
     intended method or methods of disposition by the holders of such
     Registrable Securities set forth in such Registration Statement or
     Prospectus as so supplemented;

           (iii)  notify the selling holders of Registrable Securities and the
     managing underwriters, if any, promptly, and (if requested by any such
     Person) confirm such advice in writing,

                  (A)   when the Prospectus or any supplement thereto or any
                        amendment or post-effective amendment to the
                        Registration Statement has been filed, and, with respect
                        to the Registration Statement or any post-effective
                        amendment, when the same has become effective,

                  (B)   of any request by the Commission for amendments or post-
                        effective amendments to the Registration Statement or
                        supplements to the Prospectus or for additional
                        information,

                  (C)   of the issuance by the Commission of any stop order
                        suspending the effectiveness of the Registration
                        Statement or the initiation or threatening of any
                        proceeding for such purpose,

                  (D)   if at any time the representations and warranties of ATC
                        contemplated by paragraph (xv) below cease to be true
                        and correct in all material respects,

                  (E)   of the receipt by ATC of any notification with respect
                        to the suspension of the qualification of the
                        Registrable Securities for sale in any jurisdiction or
                        the initiation or threatening of any proceeding for such
                        purpose, and

                  (F)   of the existence of any Event which results in the
                        Registration Statement, the Prospectus or any document
                        incorporated therein by reference containing an untrue
                        statement of material fact or omitting to state a
                        material fact required to be stated therein or necessary
                        to make the statements therein, in light of the
                        circumstances under which they were made, not
                        misleading;

                                      -4-
<PAGE>
 
           (iv)   use its reasonable business efforts to obtain the withdrawal
     of any order suspending the effectiveness of the Registration Statement or
     any qualification referred to in paragraphs (iii)(C) and (iii)(E) at the
     earliest possible moment;

           (v)    if requested by the managing underwriters or a holder of
     Registrable Securities being sold in connection with an underwritten
     offering, immediately incorporate in a Prospectus supplement or post-
     effective amendment to the Registration Statement such information as the
     managing underwriters or the holders of not less than a majority in value
     (based upon the proposed public offering price) of the Registrable
     Securities being sold reasonably a request to have included therein
     relating to the plan of distribution with respect to such Registrable
     Securities, including, without limitation, information with respect to the
     amount of other Registrable Securities being sold to such underwriters, the
     purchase price being paid therefor by such underwriters and with respect to
     any other terms of the underwritten (or best efforts underwritten) offering
     of the Registrable Securities to be sold in such offering; and make all
     required filings of such Prospectus supplement or post-effective amendment
     promptly after being notified of the matters to be incorporated in such
     Prospectus supplement or post-effective amendment;

           (vi)   at the request of any selling holder of Registrable
     Securities, furnish to such selling holder of Registrable Securities and
     each managing underwriter, if any, without charge, at least one signed copy
     of the Registration Statement and any post-effective amendment thereto,
     including financial statements and schedules, all documents incorporated
     therein by reference and all exhibits (including those incorporated by
     reference);

           (vii)  deliver to each selling holder of Registrable Securities and
     the underwriters, if any, without charge, as many copies of the
     Registration Statement, each Prospectus (including each preliminary
     prospectus) and any amendment or supplement thereto (in each case including
     all exhibits, except that ATC shall not be obligated to furnish any such
     selling holder more than two copies of such exhibits other than
     incorporation documents), as such persons may reasonably request, together
     with such documents incorporated by reference in such Registration
     Statement or Prospectus, and such other documents as such selling holder or
     underwriter may reasonably request in order to facilitate the disposition
     of the Registrable Securities covered by such registration statement; ATC
     consents to the use of each Prospectus or any supplement thereto by each
     selling holder of Registrable Securities and the underwriters, if any, in
     connection with the offering and sale of the Registrable Securities covered
     by each Registration Statement or any amendment thereto;

           (viii) prior to any public offering of Registrable Securities, use
     its reasonable business efforts to register or qualify or cooperate with
     the selling holders of Registrable Securities, the underwriters, if any,
     and their respective counsel in connection with the registration or
     qualification of such Registrable Securities for offer and sale under the
     securities or blue sky laws of such jurisdictions as any selling holder or
     any underwriter reasonably requests in writing and do any and all other
     acts or things necessary or advisable to enable the disposition in such
     jurisdictions of the Registrable Securities covered by the Registration
     Statement; provided, however, that ATC will not be required to qualify
     generally to do business in any jurisdiction where it is not then so
     qualified or to take any action which would subject it to general service
     of process or general taxation in any such jurisdiction where it is not
     then so subject;

                                      -5-
<PAGE>
 
          (ix)    cooperate with the selling holders of Registrable Securities
     and the underwriters, if any, to facilitate the timely preparation and
     delivery of certificates representing Registrable Securities to be sold and
     not bearing any restrictive legends; and enable such Registrable Securities
     to be in such denominations and registered in such names as the
     underwriters may reasonably request at least two (2) business days prior to
     any sale of Registrable Securities to the underwriters;

           (x)    use its reasonable business efforts to cause the Registrable
     Securities covered by the Registration Statement to be registered with or
     approved by such other governmental agencies or authorities as may be
     necessary or advisable to enable the sellers thereof or the underwriters,
     if any, to consummate the disposition of such Registrable Securities;

           (xi)   if any event contemplated by paragraph (iii) (F) above shall
     exist, prepare and furnish to such holders a post-effective amendment to
     the Registration Statement or supplement to the Prospectus or any document
     incorporated therein by reference or file any other required document so
     that, as thereafter delivered to the purchasers of the Registrable
     Securities, the Prospectus will not contain an untrue statement of a
     material fact or omit to state any material fact required to be stated
     therein or necessary to make the statements therein, in light of the
     circumstances under which they were made, not misleading;

           (xii)  cause all Registrable Securities covered by the Registration
     Statement to be listed on each securities exchange or other trading market
     on which securities of the same class are then listed or traded or, if the
     Registrable Securities are not then listed on a securities exchange, and if
     the NASD is reasonably likely to permit the inclusion of the Registrable
     Securities on NASDAQ, use its reasonable business efforts to facilitate the
     inclusion of the Registrable Securities on NASDAQ;

           (xiii) not later than the effective date of the Registration
     Statement, provide a CUSIP number for all Registrable Securities and
     provide the applicable transfer agent or agents with printed certificates
     or instruments for the Registrable Securities which are in a form eligible
     for deposit with Depository Trust Company or other transferee and otherwise
     meeting the requirements of any securities exchange or other trading market
     on which such Registrable Securities are listed or traded;

           (xiv)  enter into agreements (including underwriting agreements) and
     take all other appropriate actions in order to expedite or facilitate the
     disposition of such Registrable Securities and in such connection, whether
     or not an underwriting agreement is entered into and whether or not the
     offer and sale of the Registrable Securities is an underwritten offering:

                  (A)   make such representations and warranties to the holders
                        of such Registrable Securities and the underwriters, if
                        any, in form, substance and scope, reasonably
                        satisfactory to such holders and underwriters, as are
                        customarily made by issuers to underwriters in primary
                        underwritten offerings;

                  (B)   obtain opinions and updates thereof of counsel which
                        counsel and opinions to ATC (in form, scope and
                        substance) shall be reasonably satisfactory to

                                      -6-
<PAGE>
 
                        the underwriters, if any, and the holders of not less
                        than a majority in value (based on the proposed public
                        offering price) of the Registrable Securities being
                        sold, addressed to each selling holder and the
                        underwriters, if any, covering the matters customarily
                        covered in opinions requested in underwritten offerings
                        and such other matters as may be reasonably requested by
                        such holders and underwriters;

                  (C)   obtain so-called "cold comfort" letters and updates
                        thereof from ATC's independent public accountants
                        addressed to the selling holders of Registrable
                        Securities and the underwriters, if any, such letters to
                        be in customary form and covering matters of the type
                        customarily covered in "cold comfort" letters to
                        underwriters in connection with primary underwritten
                        offerings and such other matters as may be reasonably
                        requested by such holders and underwriters;

                  (D)   if an underwriting agreement is entered into, cause the
                        same to set forth in full the indemnification provisions
                        and procedures of Section 3 (or such other substantially
                        similar provisions and procedures as the underwriters
                        shall reasonably request) with respect to all parties to
                        be indemnified pursuant to said Section; and

                  (E)   deliver such documents and certificates as may be
                        reasonably requested by the holders of not less than a
                        majority in value (based on the proposed public offering
                        price) of the Registrable Securities being sold or the
                        underwriters, if any, to evidence compliance with the
                        provisions of this Section and with any customary
                        conditions contained in the underwriting agreement or
                        other agreement entered into by ATC.

          The requirements of subparagraphs (B), (C) and (D) of this paragraph
          (xiv) shall be complied with at the effectiveness of such Registration
          Statement, each closing under any underwriting or similar agreement as
          and to the extent required thereunder and from time to time as may
          reasonably be requested by a majority in value (based on the proposed
          public offering price) of Registrable Securities being sold pursuant
          to such Registration Statement, all in a manner consistent with
          customary industry practice;

          (xv)    make available to a representative of the holders of not less
     than a majority in value (based on the proposed public offering price) of
     the Registrable Securities, any underwriter participating in any
     disposition pursuant to such Registration Statement, and any attorney or
     accountant retained by such holders or underwriter, all financial,
     corporate and other records and documents of ATC, and cause ATC's officers,
     directors and employees to supply all information reasonably requested by
     any such representatives, underwriter, attorney or accountant in connection
     with the registration, with respect to each at such time or times as the
     person requesting such information shall reasonably determine; provided,
     however, that any records, information or documents that are designated by
     ATC in writing as confidential shall be kept confidential by such persons
     unless disclosure of such records, information or documents is required by
     court or

                                      -7-
<PAGE>
 
     administrative order or applicable law or otherwise becomes public without
     breach of the provisions of this paragraph;

          (xvi)   otherwise use its reasonable business efforts to comply with
     the Securities Act, the Exchange Act, all applicable rules and regulations
     of the Commission and all applicable state blue sky and other securities
     laws, rules and regulations, and make generally available to its security
     holders, earnings statements satisfying the provisions of Section 11(a) of
     the Securities Act, no later than thirty (30) days after the end of any 12-
     month period (or ninety (90) days if the end of such 12-month period
     coincides with the end of a fiscal quarter or fiscal year, respectively) of
     ATC (A) commencing at the end of any month in which Registrable Securities
     are sold to underwriters in an underwritten offering, or, if not sold to
     underwriters in such an offering, (B) beginning with the first month
     commencing after the effective date of the Registration Statement, which
     statements shall cover said 12-month periods;

          (xvii)  cooperate and assist in any filings required to be made with
     the NASD and in the performance of any due diligence investigation by any
     underwriter (including any "qualified independent underwriter" that is
     required to be retained in accordance with the rules and regulations of the
     NASD);

          (xviii) promptly prior to the filing of any document which is to be
     incorporated by reference into the Registration Statement or the Prospectus
     (after the initial filing of the Registration Statement) provide copies of
     such document to the selling holders of Registrable Securities, the
     underwriters, if any, and their respective counsel, make ATC's
     representatives available for discussion of such document with such persons
     and make such changes in such document prior to the filing thereof as any
     such persons may reasonably request; and

          (xix)   cooperate and assist in any filings required to be made with
     the FCC, including without limitation the obtaining of any consents of the
     FCC required in connection with any change in control of ATC.

          (d)     Restrictions on Registration.  Anything in Section 1 to the
                  ----------------------------
contrary notwithstanding, ATC shall not be required to register Registrable
Securities on behalf of any Stockholder to the following extent and subject to
the following conditions: in the case of any registration initially proposed to
be filed solely on behalf of ATC if, in the opinion of the managing underwriters
of the proposed public offering (a copy of which opinion shall have been
furnished to any Stockholder requesting registration (or each such holder if ATC
has elected not to notify the holders of Registrable Securities pursuant to the
provisions of Section 1(a) because it is not required to include any Registrable
Securities in such registration pursuant to the provisions of this Section)),
such registration (or such portion thereof as may be specified in such opinion)
would adversely affect the proposed public offering price or the plan of
distribution contemplated by the proposed ATC offering, in which event ATC shall
(unless in the opinion of such managing underwriters (a copy of which shall be
similarly furnished) to do so would materially and adversely affect the proposed
public offering price or such plan of distribution)) cause such Registration
Statement to remain in effect and to be phrased in such a manner so that the
Stockholders requesting registration thereunder may, during a period commencing
not less than sixty (60) days or more than ninety (90) days (or such other
period as such managing underwriters may approve as not so adversely affecting
the proposed public offering price

                                      -8-
<PAGE>
 
or such plan of distribution) after the closing of the sale to the underwriters
pursuant to the original distribution thereunder, offer and sell under such
Registration Statement the Registrable Securities referred to in the request of
registration pursuant to this Section 1.

     (e)  Additional Restrictions on Registration. Anything in this Agreement to
          ---------------------------------------
the contrary notwithstanding, ATC shall not be required to file a registration
statement requested pursuant to this Section 1 if ATC has furnished, to the
Stockholders requesting a registration statement to be filed, a certificate
signed by the Chief Executive Officer or the Chief Financial Officer of ATC
stating that in the good faith judgment of the signer of such certificate the
filing of a registration statement would require the disclosure of material
information that ATC has a bona fide business purpose for preserving as
confidential and that is not then otherwise required to be disclosed; provided,
however, that ATC's obligation to use its reasonable business efforts to effect
a registration pursuant to this Section 1 may not be deferred pursuant to this
paragraph (e) for more than ninety (90) days from the date of receipt of a
written request from such Stockholders, and provided further, however, that ATC
shall not utilize this right more than once during any twelve (12) month period
unless the Stockholders requesting such registration have been afforded a
reasonable period (not less than ninety (90) days) during such twelve (12) month
period to effect such registration.

     2.   Conditions to Registration.
          --------------------------

     Each Stockholder's right to have Registrable Securities included in any
Registration Statement filed by ATC in accordance with the provisions of Section
1 shall be subject to the following conditions:

          (a)  The holders on whose behalf such Registrable Securities are to be
     included shall be required to furnish ATC in a timely manner all
     information required by the applicable rules and regulations of the
     Commission concerning the proposed method of sale or other disposition of
     such Registrable Securities, the identity of and compensation to be paid to
     any proposed underwriters to be employed in connection therewith, and such
     other information as may be reasonably requested by ATC or its counsel
     properly to prepare and file such Registration Statement in accordance with
     applicable provisions of the Securities Act;

          (b)  If any such holder desires to sell and distribute Registrable
     Securities over a period of time, or from time to time, at then prevailing
     market prices, then any such holder shall execute and deliver to ATC such
     written undertakings as ATC and its counsel may reasonably request in order
     to assure full compliance with applicable provisions of the Securities Act
     and the Exchange Act;

          (c)  In the case of any underwritten offering on behalf of the holders
     of Registrable Securities pursuant to the provisions of Section 1(b), the
     managing underwriters (other than CSFB or its affiliates) shall be subject
     to the approval of ATC, such approval not to be unreasonably withheld,
     delayed or conditioned;

          (d)  In the case of any registration requested pursuant to the
     provisions of Section 1(a), the offering price for any Registrable
     Securities to be so registered shall be no less than for any securities of
     the same class then to be registered for sale for the account of ATC or
     other security

                                      -9-
<PAGE>
 
     holders, unless such Registrable Securities are to be offered from time to
     time based on the prevailing market price;

          (e)  Upon receipt of any notice from ATC of the existence of any event
     of the nature referred to in paragraph (iii) of Section 1(c), such holder
     will forthwith discontinue disposition of Registrable Securities until such
     holder's receipt of the copies of the supplemented or amended Prospectus
     contemplated by such paragraph, or until it is advised in writing by ATC
     that the use of the Prospectus may be resumed, and has received copies of
     any additional or supplemental filings which are incorporated by reference
     in the Prospectus, and, if so directed by ATC, such holder will deliver to
     ATC all copies, other than permanent file copies then in such holder's
     possession, of the Prospectus covering such Registrable Securities current
     at the time of receipt of such notice;

          (f)  In the event any filing with or consent of the FCC is required,
     cooperate and assist in any such filings, including without limitation
     providing all information required in obtaining any consents of the FCC
     required in connection with any change in control of ATC; and

          (g)  The holders of Registrable Securities requesting registration
     shall pay all Registration Expenses in connection with such registration
     pursuant to the provisions of this Section.

     3.   Indemnification.
          ---------------

     (a)  Indemnification by ATC.  In the event of the registration of any
          ----------------------
Registrable Securities under the Securities Act pursuant to the provisions
hereof, ATC will, to the extent permitted by Applicable Law, indemnify and hold
harmless each Stockholder on whose behalf such Registrable Securities shall have
been registered, its partners, trustees, advisory committee members, directors,
officers, employees, representatives and agents, each underwriter, broker and
dealer, if any, who participates in the offering or sale of such Registrable
Securities, and each other Person, if any, who controls such Stockholder or any
such underwriter, broker or dealer within the meaning of the Securities Act or
the Exchange Act (each such person being hereinafter sometimes referred to as an
"indemnified person"), from and against any Claims, joint or several, to which
such indemnified person may become subject, including without limitation under
the Securities Act, the Exchange Act or any state securities or blue sky law,
insofar as such Claims arise out of or are based upon any untrue statement or
alleged untrue statement of any material fact contained or incorporated by
reference in any Registration Statement or Prospectus or any amendment or
supplement thereto or in any preliminary prospectus, or any document
incorporated by reference therein, or arise out of or based upon the omission or
alleged omission to state therein a material fact required to be stated therein
or necessary to make the statements therein, in light of the circumstances under
which they were made, not misleading, and will reimburse each such indemnified
person for any legal or any other expenses reasonably incurred by such
indemnified person in connection with investigating or defending, settling or
satisfying any such Claim; provided, however, that ATC will not be liable in any
such case to the extent that any such Claim arises out of or is based upon an
untrue statement or alleged untrue statement or omission or alleged omission
made or incorporated by reference in the Registration Statement, Prospectus,
amendment or supplement in reliance upon and in conformity with written
information furnished to ATC by such indemnified person specifically stating
that it is for use in preparation thereof. Such indemnity shall remain in full
force and effect regardless 

                                      -10-
<PAGE>
 
of any investigation made by or on behalf of such
indemnified person and shall survive the transfer of such Registrable Securities
by such Stockholder.

     (b)  Indemnification by Holders of Registrable Securities. In the event
          ----------------------------------------------------
of the registration of any Registrable Securities under the Securities Act
pursuant to the provisions hereof, each Stockholder on whose behalf such
Registrable Securities shall have been registered will, to the extent permitted
by Applicable Law, severally but not jointly, indemnify and hold harmless, ATC,
each director of ATC, each officer of ATC who signs the registration statement,
each underwriter, broker and dealer, if any, who participates in the offering
and sale of such Registrable Securities and each other Person, if any, who
controls ATC or any such underwriter, broker or dealer within the meaning of the
Securities Act or the Exchange Act (each such person including without
limitation ATC being hereinafter sometimes referred to as an "indemnified
person"), against any Claims, joint or several, to which such indemnified person
may become subject, including without limitation under the Securities Act, the
Exchange Act or any state securities or blue sky law, insofar as such Claims
arise out of or are based upon any untrue statement or alleged untrue statement
of a material fact contained or incorporated by reference in any Registration
Statement or Prospectus or any amendment or supplement thereto or any document
incorporated by reference therein, or arise out of or are based upon the
omission or alleged omission to state therein a material fact required to be
stated therein or necessary to make the statements therein, in light of the
circumstances under which they were made, not misleading, provided that such
untrue statement or alleged untrue statement or omission or alleged omission has
been made or incorporated therein in reliance upon and in conformity with
written information furnished to ATC by such Stockholder specifically stating
that it is for use in preparation thereof, and will reimburse each such
indemnified person for any legal or any other expenses reasonably incurred by
ATC or such indemnified person in connection with investigating or defending,
settling or satisfying any such Claim. Such indemnity shall remain in full force
and effect regardless of any investigation made by or on behalf of such
indemnified person and shall survive the transfer of such Registrable Securities
by such Stockholder. In no event shall the liability of any such Stockholder
hereunder be greater in amount than the dollar amount of the proceeds received
by such Stockholder upon the sale of the Registrable Securities giving rise to
such indemnification obligation.

     (c)  Procedure.  Promptly after receipt by an indemnified party of notice
          ---------
of the commencement of any action (including any governmental investigation or
inquiry), such indemnified party will, if a claim in respect thereof is to be
made against an indemnifying party, give written notice to such indemnifying
party of the commencement thereof, but the omission so to notify the
indemnifying party will not relieve it from any liability which it may have to
any indemnified party otherwise than pursuant to the provisions of this Section
and then only to the extent such indemnifying party has been prejudiced, or
otherwise adversely affected thereby and in no event shall such failure relieve
the indemnifying party from any other liability which it may have to the
indemnified party. In case any such action is brought against any indemnified
party, and it notifies an indemnifying party of the commencement thereof, the
indemnifying party will be entitled to participate in, and to the extent that it
may wish, jointly with any other indemnifying party similarly notified, to
assume the defense thereof, with counsel reasonably satisfactory to such
indemnified party, and after notice from the indemnifying party to such
indemnified party, the indemnifying party shall not, except as hereinafter
provided, be responsible for any legal or other expenses subsequently incurred
by such indemnified party in connection with the defense thereof, other than
reasonable cost of investigation. No indemnifying party will consent to entry of
any judgment or enter into any settlement which does not 

                                      -11-
<PAGE>
 
include as an unconditional term thereof the giving by the claimant or plaintiff
to such indemnified party of a release from all liability in respect to such
Claim.

     Such indemnified party shall have the right to employ separate counsel in
any such action and to participate in the defense thereof, but the fees and
expenses of such counsel shall be the expense of such indemnified party, unless
(i) the indemnifying party has agreed to pay such fees and expenses, (ii) the
indemnifying party shall have failed to assume the defense of such action or
proceeding or has failed to employ counsel reasonably satisfactory to such
indemnified party in any such action or proceeding, or (iii) the named parties
to any such action or proceeding (including any impleaded parties) include both
such indemnified party and the indemnifying party, and such indemnified party
shall have been advised in writing by counsel that representation of both
parties by the same counsel would be inappropriate due to actual or potential
material differing interests between them (in which case, if such indemnified
party notifies the indemnifying party in writing that it elects to employ
separate counsel at the expense of the indemnifying party, the indemnifying
party shall not have the right to assume the defense of such action or
proceeding on behalf of such indemnified party, it being understood, however,
that the indemnifying party shall not, in connection with any one such action or
proceeding or separate but substantially similar or related actions or
proceedings in the same jurisdiction arising out of the same general allegations
or circumstances, be liable for the fees and expenses of more than one separate
firm of attorneys at any time for such indemnified party and any other
indemnified parties, which firm shall be designated in writing by such
indemnified parties). The indemnifying party shall not be liable for any
settlement of any such action or proceeding effected without its written
consent, but if settled with its written consent, or if there be a final
judgment for the plaintiff in any such action or proceeding, the indemnifying
party agrees to indemnify and hold harmless such indemnified parties from and
against any loss or liability by reason of such settlement or judgment.

     (d)  Contribution.  If the indemnification provided for in this Section
          ------------
or in Section 4 is unavailable, because prohibited or restricted by Applicable
Law, to a party that would have been an indemnified party under either such
Section in respect of any Claims referred to therein, then each party that would
have been an indemnifying party thereunder shall, in lieu of indemnifying such
indemnified party, contribute to the amount paid or payable by such indemnified
party as a result of such Claims in such proportion as is appropriate to reflect
the relative fault of the indemnifying party on the one hand and such
indemnified party on the other in connection with the statement or omission
which resulted in such Claims, as well as any other relevant equitable
considerations. The relative fault shall be determined by reference to, among
other things, whether the untrue or alleged untrue statement of a material fact
or the omission or alleged omission to state a material fact relates to
information supplied by the indemnifying party or such indemnified party and the
parties' relative intent, knowledge, access to information and opportunity to
correct or prevent such statement or omission. Notwithstanding the provisions of
this Section, a holder of Registrable Securities shall not, as an indemnified
party, be required to contribute any amount in excess of the amount by which the
total price at which the Registrable Securities sold by such indemnified party
or its Affiliates and distributed to the public were offered to the public
exceeds the amount of any damages which such indemnified party or its Affiliates
have otherwise been required to pay by reason of such untrue or alleged untrue
statement or omission or alleged omission. ATC and each holder of Registrable
Securities agrees that it would not be just and equitable if contribution
pursuant to this Section were determined by pro rata allocation or by any other
method of allocation which does not take account of the equitable considerations
referred to above in this Section. The amount paid or payable by an indemnified
party as a 

                                      -12-
<PAGE>
 
result of the Claims referred to above in this Section or Section 4 shall
include any legal or other expenses reasonably incurred by such indemnified
party in connection with investigation or defending any such action or claim
(which shall be limited as provided in Section 3(c) if the indemnifying party
has assumed the defense of any such action in accordance with the provisions
thereof). The obligations of each Stockholder under this Section 3(d) are
several and not joint.

     No person guilty of fraudulent misrepresentation (within the meaning of
Section 11(f) of the Securities Act) shall be entitled to contribution from any
person who was not guilty of such fraudulent misrepresentation.

     Indemnification or, if appropriate, contribution, similar to that specified
in the preceding provisions of this Section (with appropriate modifications)
shall be given by ATC and each seller of Registrable Securities with respect to
any required registration or other qualification of Registrable Securities under
any Applicable Law other than the Securities Act.

     In the event of any underwritten offering of Registrable Securities under
the Securities Act pursuant to the provisions of Section 1, ATC and each
Stockholder on whose behalf Registrable Securities shall have been registered
agree to enter into an underwriting agreement, in standard form, with the
underwriters, which underwriting agreement may contain additional provisions
with respect to indemnification and contribution in lieu of the provisions of
this Section.

     4.   Exchange Act Registration.
          -------------------------

     ATC covenants and agrees that, at its expense, until such time as the
Stockholders no longer hold any Registrable Securities:

          (a)   it will, if required by law, maintain a registration statement
     (containing such information and documents as the Commission shall specify)
     with respect to the ATC Common Stock under Section 12(b) or 12(g) of the
     Exchange Act effective and will file on time such information, documents
     and reports as the Commission may require or prescribe for companies whose
     stock has been registered pursuant to said Section 12(b) or 12(g);

          (b)   it will, if a registration statement with respect to the ATC
     Common Stock under Section 12(b) or Section 12(g) is effective, upon the
     request of any Stockholder, make whatever other filings with the Commission
     or otherwise make generally available to the public such financial and
     other information as any Stockholder may deem necessary or advisable in
     order to enable him to be permitted to sell shares of ATC Common Stock
     pursuant to the provisions of Rule 144 promulgated under the Securities Act
     (or any successor rule or regulation thereto or any statute hereafter
     adopted to replace or to establish the exemption that is now covered by
     said Rule 144);

          (c)   it will, if not subject to Section 13 to 15(d) of the Exchange
     Act, upon the request of any Significant Stockholder made on or after
     December 31, 1998, make publicly available the information specified in
     subparagraph (c)(2) of said Rule 144, and will take such further action as
     any Stockholder may reasonably request, all to the extent required from
     time to time to enable such 

                                      -13-
<PAGE>
 
     Stockholder to sell Registrable Securities without registration under the
     Securities Act within the limitation of the exemptions provided by said
     Rule 144 (or any successor rule or regulation to either thereof or any
     statute hereafter adopted to replace or to establish the exemption that is
     now covered by said Rule 144); and

          (d)   it will, if not subject to Section 13 to 15(d) of the Exchange
     Act, upon the request of any Stockholder agree to furnish to a prospective
     purchaser (subject to the execution by it of a confidentiality agreement in
     form, scope and substance reasonably satisfactory to ATC) the information
     specified in subparagraph (d)(4) of Rule 144A promulgated under the
     Securities Act (or any successor rule or regulation thereto or any statute
     hereafter adopted to replace or to establish the exemption that is now
     covered by said Rule 144A), and will take such further action as any
     Stockholder may reasonably request, all to the extent required from time to
     time to enable such Stockholder to sell Registrable Securities without
     registration under the Securities Act within the limitation of the
     exemptions provided by said Rule 144A (or any successor rule or regulation
     thereto or any statute hereafter adopted to replace or to establish the
     exemption that is now covered by said Rule 144A); and

          (e)   upon the request of any Stockholder, it will deliver to such
     Stockholder a written statement as to whether it has complied with the
     requirements of this Section.

     ATC represents and warrants that any such registration statement or any
information, documents or report filed with the Commission in connection
therewith or any information so made public shall not contain any untrue
statement of a material fact or omit to state a material fact required to be
stated therein or necessary in order to make the statements contained therein,
in light of the circumstances under which they were made, not misleading. ATC
shall, to the extent permitted by Applicable Law, indemnify and hold harmless
(or to the extent the same is not enforceable, make contribution to) the
Stockholders, their partners, trustees, advisory committee members, officers,
directors, employees, representatives and agents, each broker, dealer or
underwriter (within the meaning of the Securities Act) acting for any
Stockholder in connection with any offering or sale by such Stockholder of
Registrable Securities or any person, firm or corporation controlling (within
the meaning of Section 15 of the Securities Act or Section 20 of the Exchange
Act) such Stockholder or any such broker, dealer or underwriter from and against
any and all Claims arising out of or resulting from any breach of the foregoing
representation or warranty, all on terms and conditions comparable to those set
forth in Section 3; provided, however, that ATC shall be given written notice
and an opportunity to participate in, and, to the extent that it may wish, to
assume on terms and conditions comparable to those set forth in Section 3, the
defense thereof.

     5.   Termination of Registration Obligations.
          ---------------------------------------

     The obligations of ATC to any Stockholder with respect to its rights of
registration provided for in Section 1:

          (a)   shall continue until such time as Sullivan & Worcester LLP, or
     other counsel for ATC knowledgeable in securities law matters and
     reasonably acceptable to such Stockholder has delivered a written opinion
     to ATC and such Stockholder to the effect that either (i) such

                                      -14-
<PAGE>
 
     Stockholder has no further obligation to comply with the registration
     requirements of the Securities Act or to deliver a prospectus meeting the
     requirements of Section 10(a)(3) of the Securities Act in connection with
     further sales by such Stockholder of Registrable Securities or (ii) such
     Stockholder is able to sell all of the Registrable Securities owned by it
     pursuant to the provisions of Rule 144 under the Securities Act in a three-
     month period; and

          (b)   shall not apply to any proposed sales or other dispositions or
     offers therefor of any Registrable Securities with respect to which
     Sullivan & Worcester LLP, or other counsel for ATC knowledgeable in
     securities law matters and reasonably acceptable to such Stockholder has
     delivered a written opinion to ATC and such Stockholder proposing to make
     such offer, sale or other disposition to the effect that such Stockholder
     has no obligation to comply with the registration requirements of the
     Securities Act or to deliver a prospectus meeting the requirements of
     Section 10(a)(3) of the Securities Act.

     Any such opinion (a copy of which shall be addressed to such Stockholder)
shall be reasonably satisfactory (in the case of such opinion as to form, scope
and substance) to such Stockholder.

     ATC shall, to the extent permitted by Applicable Law, indemnify and hold
harmless each Stockholder, its partners, trustees, advisory committee members,
officers, directors, employees, representatives and agents and each person, if
any, who controls such Stockholder within the meaning of either Section 15 of
the Securities Act or Section 20 of the Exchange Act, against any Claims to
which such Stockholder, or such partners, trustees, advisory committee members,
officers, directors, employees, representatives and agents or controlling
persons may become subject under the Securities Act, the Exchange Act or
otherwise, insofar as such Claims arise out of or are based upon the failure to
register the Registrable Securities because of the invocation by ATC of the
provisions of this Section under the Securities Act, all on terms and conditions
comparable to those set forth in Section 3; provided, however, that ATC shall be
given written notice and an opportunity to participate in, and to the extent
that it may wish, to assume, on terms and conditions comparable to those set
forth in Section 3, the defense thereof.

     The indemnification and contributions provisions of Sections 3 and 4 and
this Section, and the obligations of each Stockholder pursuant to the provisions
of Section 9, shall survive any termination of ATC's obligations pursuant to
this Section.

     6.   Registration Rights of Others.
          -----------------------------

     ATC represents and warrants that it has not previously entered into any
agreement with respect to its securities granting any registration rights to any
Person other than the 1998 Registration Rights Agreement.

     7.   Mergers, etc.
          ------------

     In addition to any other restrictions on mergers, consolidations and
reorganizations contained in the Restated Certificate of Incorporation, by-laws
or agreements of ATC, ATC covenants and agrees that it shall not, directly or
indirectly, enter into any merger, consolidation, sale of all or substantially
all of its assets or 

                                      -15-
<PAGE>
 
business, liquidation, dissolution or reorganization in which ATC shall not be
the surviving corporation unless the surviving corporation shall, prior to such
merger, consolidation or reorganization, agree in a writing to assume all of the
obligations of ATC under this Agreement, and for that purpose references
hereunder to "Registrable Securities" shall be deemed to include the securities
which such holders would be entitled to receive in exchange for Registrable
Securities pursuant to any such merger, consolidation, sale of all or
substantially all of its assets or business, liquidation, dissolution or
reorganization.

     8.   Annual and Quarterly Reports; Other Information.
          -----------------------------------------------

     ATC will deliver to each Stockholder so long as such Stockholder holds any
Registrable Securities:

          (a)  as soon as practicable after the end of each fiscal year and each
     quarter, audited annual and unaudited consolidated quarterly financial
     statements of ATC, including a consolidated balance sheet, a consolidated
     statement of operations, and a consolidated statement of cash flow, for
     such year or quarter, all prepared in accordance with generally accepted
     accounting principles;

          (b)  as soon as available, copies of all documents filed with the
     Commission; and

          (c)  such other financial and other information as may, from time to
     time, be reasonably requested by any Significant Stockholder.

     9.   Withdrawals.
          -----------

     Any Stockholder may at any time withdraw any request made pursuant to
Section 1 for registration of its Registrable Securities. Notwithstanding
anything in the foregoing provisions of this Section to the contrary, the
provisions of this Section shall not be applicable in the event that any such
withdrawal or withdrawals resulting in such termination is or are effected on
account of (a) ATC's failure to disclose any material fact required to be
disclosed in the registration statement or any prospectus relating to such
offering or (b) any material adverse change in ATC, its business, assets or
condition (financial or other).

     10.  Definitions.
          -----------

     As used herein, unless the context otherwise requires, the terms (or any
variant in the form thereof) set forth in this Agreement shall have the
respective meanings so set forth. Terms defined in the singular shall have a
comparable meaning when used in the plural, and vice versa, and the reference to
any gender shall be deemed to include all genders. Unless otherwise defined or
the context otherwise clearly requires, terms for which meanings are provided in
this Agreement shall have such meanings when used in each agreement, notice,
certificate, communication, opinion or other document executed or required to be
executed pursuant hereto or thereto or otherwise delivered, from time to time,
pursuant hereto or thereto.

     "AFFILIATE" of any Person shall mean any Person which, directly or
indirectly, owns or controls, is under common ownership or control with, or is
owned or controlled by, such Person. A Person shall be deemed to be "controlled
by" any other Person if such other Person possesses, directly or indirectly,
power to direct or cause the direction of the management or policies of such
Person or the disposition of its assets

                                      -16-
<PAGE>
 
or property, whether by stock, equity or other ownership, contract, arrangement
or understanding, or otherwise.

     "AGREEMENT" is defined in the first paragraph.

     "APPLICABLE LAW" shall mean any Law of any Authority, whether domestic or
foreign, including without limitation all federal and state Laws, to which the
Person in question is subject or by which it or any of its business or
operations is subject or any of its property is bound.

     "ATC" is defined in the first paragraph.

     "AUTHORITY" shall mean any governmental or quasi-governmental authority,
whether executive, legislative, judicial, administrative or other, or any
combination thereof, including without limitation any federal, state,
territorial, county, municipal or other government or governmental or quasi-
governmental agency, arbitrator, board, body, branch, bureau or comparable
agency or Entity, commission, corporation, court, department, instrumentality,
mediator, panel, system or other political unit or subdivision or other Entity
of any of the foregoing, whether domestic or foreign.

     "CLAIMS" shall mean, with respect to any Person, any and all debts,
liabilities, obligations, losses, damages, deficiencies, assessments and
penalties of or against such Person, together with all Legal Actions, pending or
threatened, claims and judgments of whatever kind and nature relating thereto,
and all fees, costs, expenses and disbursements (including without limitation
reasonable attorneys' and other legal fees, costs and expenses) relating to any
of the foregoing.

     "COMMON STOCK", "CLASS A COMMON", "CLASS B COMMON" or "CLASS C COMMON",
shall mean those respective securities described in the Restated Certificate of
Incorporation of ATC.

     "COMMISSION" shall mean the Securities and Exchange Commission or any
successor Authority.

     "EXCLUDED OFFERING" shall mean (a) an offering relating solely to dividend
reinvestment plans or stock option or other employee benefit plans, (b) any
merger, consolidation or acquisition, (c) any exchange or tender offer, whether
with existing security holders of ATC or any other Person, or (d) a firm
underwritten offering relating solely to convertible securities or units
consisting of securities senior to Common Stock and warrants, options and rights
to acquire Common Stock in which the managing underwriters shall have objected
to the inclusion of any Registrable Securities.

     "ENTITY" shall mean any corporation, firm, unincorporated organization,
association, partnership, a trust (inter vivos or testamentary), an estate of a
deceased, insane or incompetent individual, business trust, joint stock company,
joint venture or other organization, entity or business, whether acting in an
individual, fiduciary or other capacity, or any Authority.

     "EVENT" shall mean the existence or occurrence of any act, action,
activity, circumstances, condition, event, fact, failure to act, omission,
incident or practice, or any set or combination of any of the foregoing.

                                      -17-
<PAGE>
 
     "EXCHANGE ACT" shall mean the Securities Exchange Act of 1934, and the
rules and regulations of the Commission thereunder, all as from time to time in
effect, or any successor law, rules or regulations, and any reference to any
statutory or regulatory provision shall be deemed to be a reference to any
successor statutory or regulatory provision.

     "FCC" shall mean the Federal Communications Commission or any successor
Authority.

     "LAW" shall mean any (a) administrative, judicial, legislative or other
action, code, consent decree, constitution, decree, directive, enactment,
finding, law, injunction, interpretation, judgment, order, ordinance, policy
statement, proclamation, promulgation, regulation, requirement, rule, rule of
law, rule of public policy, settlement agreement, statute, or writ of any
Authority, domestic or foreign; (b) the common law, or other legal precedent; or
(c) arbitrator's, mediator's or referee's award, decision, finding or
recommendation.

     "LEGAL ACTION" shall mean, with respect to any Person, any and all
litigation or legal or other actions, arbitrations, counterclaims,
investigations, proceedings, requests for material information by or pursuant to
the order of any Authority or suits, at law or in arbitration, equity or
admiralty, whether or not purported to be brought on behalf of such Person,
affecting such Person or any of such Person's business, property or assets.

     "NASD" shall mean the National Association of Securities Dealers, Inc.

     "NASDAQ" shall mean the automatic quotation system of NASD.

     "PERSON" shall mean any natural individual or any Entity.

     "PROSPECTUS" shall mean each prospectus included in any Registration
Statement, as amended or supplemented by any prospectus supplement with respect
to the terms of the offering of any portion of the Registrable Securities
covered by the Registration Statement and by all other amendments and
supplements to the prospectus, including each preliminary prospectus and post-
effective amendments and all material incorporated by reference in such
prospectus.

     "REGISTRABLE SECURITIES" shall mean (a) all shares of Class A Common Stock
acquired by CSFB pursuant to the ATC Stock Purchase Agreement, and (b) all
shares of Common Stock of whatever series or class or other equity securities of
ATC derived from the Registrable Securities, whether as a result of merger,
consolidation, stock split, stock dividend, stock distribution, stock
combination, recapitalization or similar event.

     "REGISTRATION EXPENSES" shall mean all (or where appropriate any one or
more) of the following:

          (a)  all registration, filing and listing fees;

          (b)  fees and expenses of compliance with securities or blue sky laws
     (including without limitation reasonable fees and disbursements of counsel
     for the underwriters or selling holders in connection with blue sky and
     state securities qualifications of the Registrable Securities under the

                                      -18-
<PAGE>
 
     laws of such jurisdictions as the managing underwriters or the holders of
     not less than a majority in value (based on the proposed public offering
     price) of the Registrable Securities being sold may designate);

          (c)  printing (including without limitation expenses of printing or
     engraving certificates for the Registrable Securities in a form eligible
     for deposit with Depositary Trust Company and otherwise meeting the
     requirements of any securities exchange on which they are listed and of
     printing Prospectuses), word processing, messenger, telephone and delivery
     expenses;

          (d)  fees and disbursements of counsel for ATC, and reasonable fees
     and disbursements of counsel for the underwriters and for the selling
     holders of the Registrable Securities in accordance with the provisions of
     Section 1(c)(xiv) (subject to any provisions to the contrary in this
     Agreement);

          (e)  fees and disbursements of all independent public accountants of
     ATC (including without limitation the expenses of any annual or special
     audit and "cold comfort" letters required by the provisions of this
     Agreement);

          (f)  fees and disbursements of underwriters (excluding discounts,
     commissions or fees of underwriters), selling brokers, dealer managers or
     similar securities industry professionals relating to the distribution of
     the Registrable Securities or legal expenses of any Person other than ATC,
     the underwriters and the selling holders;

          (g)  securities act liability insurance if ATC so desires or if the
     underwriters or the holders of not less than a majority in value (based on
     the proposed public offering price) of the Registrable Securities being
     sold so require;

          (h)  fees and expenses of other Persons, including any experts,
     retained by ATC;

          (i)  fees and expenses incurred in connection with the listing of the
     Registrable Securities on each securities exchange on which securities of
     the same class are then listed;

          (j)  fees and expenses associated with any NASD filing required to be
     made in connection with any Registration Statement, including, if
     applicable, the fees and expenses of any "qualified independent
     underwriter" (and its counsel) that is required to be retained in
     accordance with the rules and regulations of the NASD;

          (k)  all other third party costs and expenses normally associated with
     the issuance and sale of newly issued public securities.

     "REGISTRATION STATEMENT" shall mean any registration statement of ATC which
covers Registrable Securities pursuant to the provisions of this Agreement,
including the Prospectus, amendments, including post-effective amendments to
such registration statement, and supplements to such Prospectus and all exhibits
and all material incorporated by reference in such registration statement.

                                      -19-
<PAGE>
 
     "SECURITIES ACT" shall mean the Securities Act of 1933, and the rules and
regulations of the Commission thereunder, all as from time to time in effect, or
any successor law, rules or regulations, and any reference to any statutory or
regulatory provision shall be deemed to be a reference to any successor
statutory or regulatory provision.

     "SIGNIFICANT STOCKHOLDER" shall mean CSFB (regardless of amount owned) or
any other Stockholder, or group of Stockholders acting together, which owns
shares of Common Stock with a market value of not less than $10,000,000;
provided, however, that notwithstanding the foregoing, in the event ATC is, at
the time of any request made pursuant to the provisions of Section 1(b),
eligible to file a Registration Statement on Form S-3 (or any successor form)
with respect to the proposed disposition of the Registrable Securities with
respect to which such request has been made, and such form is acceptable to the
holders making such request, the minimum market value of the Registrable
Securities shall be not less than $5,000,000.

     "STOCKHOLDERS" shall mean CSFB and any persons who hereafter become
assignees pursuant to Section 11(a).

     "SUBSIDIARY" shall mean, with respect to any Person, any Entity a majority
of the capital stock ordinarily entitled to vote for the election of directors,
or if no such voting stock is outstanding a majority of the equity interests, of
which is owned directly or indirectly by such Person or any Subsidiary of such
Person.

     11.  Miscellaneous.
          -------------

     (a)  Assignment; Successors and Assigns. In the event that ATC shall be
          ----------------------------------
merged with, or consolidated into, any other Entity or in the event that it
shall sell and transfer substantially all of its assets to another Entity, the
terms of this Agreement shall inure to the benefit of, and be assumed by, the
Entity resulting from such merger or consolidation, or to which ATC's assets
shall be sold and transferred. Anything in this Agreement to the contrary
notwithstanding, the term "Stockholders" as used in this Agreement shall be
deemed to include the holders from time to time of any of the Registrable
Securities, whether or not they become parties to this Agreement, except for
holders who have acquired Registrable Securities in connection with an offering
registered under the Securities Act or pursuant to sales made in accordance with
Rule 144 (or any successor rule or regulation or statute in substitution
therefor). The rights to cause ATC to register Registrable Securities pursuant
to Section 1 may be assigned in connection with any transfer or assignment by a
holder of Registrable Securities; provided, however, that (i) such transfer may
otherwise be effected in accordance with applicable securities laws and (ii)
such transfer is effected in compliance with the restrictions on transfer
contained in any agreement between ATC and such holder. ATC's obligations under
this Agreement shall not be assigned, and its duties under this Agreement shall
not be delegated, except as provided in the first sentence of this Section.
Nothing in this Agreement expressed or implied is intended to and shall not be
construed to confer upon or create in any Person (other than the parties hereto
and their permitted successors and assigns) any rights or remedies under or by
reason of this Agreement, including without limitation any rights to enforce
this Agreement.

     (b)  Specific Performance; Other Rights and Remedies. Each party
          -----------------------------------------------
recognizes and agrees that the other parties' remedies at law for any breach of
the provisions of this Agreement would be inadequate 

                                      -20-
<PAGE>
 
and agrees that for breach of such provisions, each such party shall, in
addition to such other remedies as may be available to it at law or in equity or
as provided in this Agreement, be entitled to injunctive relief and to enforce
its rights by an action for specific performance to the extent permitted by Law.
Each party hereby waives any requirement for security or the posting of any bond
or other surety in connection with any temporary or permanent award of
injunctive, mandatory or other equitable relief. Nothing herein contained shall
be construed as prohibiting any party from pursuing any other remedies available
to it for such breach or threatened breach, including without limitation the
recovery of damages.

     (c)  Expenses. CSFB shall pay the expenses incident to the negotiation,
          --------
preparation, performance and enforcement of this Agreement (including all fees
and expenses of its and ATC's counsel, accountants and other consultants,
advisors and representatives for all activities of such persons undertaken
pursuant to this Agreement), except to the extent otherwise specifically set
forth in this Agreement.

     (d)  Entire Agreement. This Agreement constitutes the entire agreement
          ----------------
among the parties with respect to the subject matter hereof and supersedes all
prior agreements, arrangements, covenants, promises, conditions, understandings,
inducements, representations and negotiations, expressed or implied, oral or
written, among them as to such subject matter.

     (e)  Waivers; Amendments. Notwithstanding anything in this Agreement to the
          -------------------
contrary, amendments to and modifications of this Agreement may be made,
required consents and approvals may be granted, compliance with any term,
covenant, agreement, condition or other provision set forth herein may be
omitted or waived, either generally or in a particular instance and either
retroactively or prospectively with, but only with, the written consent of ATC
(to the extent it is entitled to the benefit thereof) and (i) with respect to
the rights of the Stockholders set forth in Section 1(b), including without
limitation the definition of Significant Stockholder, two-thirds (2/3) in
interest of the Stockholders, and (ii) with respect to all other rights and
obligations of the Stockholders, a majority in interest of the Stockholders (to
the extent they are entitled to the benefit thereof or obligated thereby);
provided, however, that in the event any such amendment, modification, consent,
approval or waiver shall be for the benefit of or materially adverse to less
than all of the Stockholders, such amendment, modification, consent, approval or
waiver shall require a majority in interest of those Stockholders who are not so
benefitted or who are so materially adversely affected.

     (f)  Notices. All notices and other communications which by any provision
          -------
of this Agreement are required or permitted to be given shall be given in
writing and shall be (a) mailed by first-class or express mail, postage prepaid,
(b) sent by telex, telegram, telecopy or other form of rapid transmission,
confirmed by mailing (by first class or express mail, postage prepaid) written
confirmation at substantially the same time as such rapid transmission, or (c)
personally delivered to the receiving party (which if other than an individual
shall be an officer or other responsible party of the receiving party). All such
notices and communications shall be mailed, sent or delivered as follows:

     If to American Tower Corporation, at

                    116 Huntington Avenue
                    Boston, MA 02116

                                      -21-
<PAGE>
 
                    Attention: Steven B. Dodge, Chairman of the Board and Chief 
                               Executive Officer
                    Facsimile: (617) 375-7575

          with a copy to:

                    Sullivan & Worcester LLP
                    One Post Office Square
                    Boston, MA  02109
                    Attention:  Norman A. Bikales, Esq.
                    Facsimile:  (617) 338-2880

     If to CSFB:

                    11 Madison Avenue
                    New York, New York  10010
                    Attention:  Kristin M. Allen, Managing Director
                    Facsimile:  (212) 325-8021

          with a copy to:

                    Sullivan &Cromwell
                    125 Broad Street
                    New York, NY  10004
                    Attention:  John T. Bostelman
                    Facsimile:  (212) 558-3588

     (g)  Severability. If any provision of this Agreement shall be held or
          ------------
deemed to be, or shall in fact be, invalid, inoperative, illegal or
unenforceable as applied to any particular case in any jurisdiction or
jurisdictions, or in all jurisdictions or in all cases, because of the
conflicting of any provision with any constitution or statute or rule of public
policy or for any other reason, such circumstance shall not have the effect of
rendering the provision or provisions in question invalid, inoperative, illegal
or unenforceable in any other jurisdiction or in any other case or circumstance
or of rendering any other provision or provisions herein contained invalid,
inoperative, illegal or unenforceable to the extent that such other provisions
are not themselves actually in conflict with such constitution, statute or rule
of public policy, but this Agreement shall be reformed and construed in any such
jurisdiction or case as if such invalid, inoperative, illegal or unenforceable
provision had never been contained herein and such provision reformed so that it
would be valid, operative and enforceable to the maximum extent permitted in
such jurisdiction or in such case, except when such reformation and construction
could operate as an undue hardship on either party, or constitute a substantial
deviation from the general intent and purpose of such party as reflected in this
Agreement. The parties shall endeavor in good faith negotiations to replace the
invalid, inoperative, illegal or unenforceable provisions with valid, operative,
legal and enforceable provisions the economic effect of which comes as close as
possible to that of the invalid, inoperative, illegal or unenforceable
provisions.

                                      -22-
<PAGE>
 
     (h)  Counterparts. This Agreement may be executed in several counterparts,
          ------------
each of which shall be deemed an original, but all of which together shall
constitute one and the same instrument, binding upon all the parties hereto. In
pleading or proving any provision of this Agreement, it shall not be necessary
to produce more than one of such counterparts.

     (i)  Section Headings. The headings contained in this Agreement are for
          ----------------
reference purposes only and shall not in any way affect the meaning or
interpretation of this Agreement.

     (j)  Governing Law. The validity, interpretation, construction and
          -------------
performance of this Agreement shall be governed by the applicable laws of the
United States of America and the domestic substantive laws of the State of New
York without giving effect to any choice or conflict of laws provision or rule
that would cause the application of domestic substantive laws of any other
jurisdiction.

     (k)  Further Acts. Each party agrees that at any time, and from time to
          ------------
time, before and after the consummation of the transactions contemplated by this
Agreement, it will do all such things and execute and deliver all such
agreements, assignments, instruments, other documents and assurances, as any
other party or its counsel reasonably deems necessary or desirable in order to
carry out the terms and conditions of this Agreement and the transactions
contemplated hereby or to facilitate the enjoyment of any of the rights created
hereby or to be created hereunder.

                                      -23-
<PAGE>
 
     IN WITNESS WHEREOF, the parties have executed or caused to be executed this
Agreement as of the date set forth in the first paragraph hereof.

                     American Tower Corporation


                     By:__________________________________
                        Name: Steven B. Dodge
                        Title: Chairman of the Board and Chief Executive Officer


                     Credit Suisse First Boston Corporation


                     By:______________________________________
                        Name:
                        Title:

                                      -24-


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