NORTHERN ILLINOIS GAS CO /IL/ /NEW/
S-3, 1997-12-18
NATURAL GAS TRANSMISSION
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<PAGE>   1
 
   AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON DECEMBER 18, 1997
 
                                                      REGISTRATION NO. 333-
================================================================================
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------
 
                                    FORM S-3
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                            ------------------------
 
                         NORTHERN ILLINOIS GAS COMPANY
                     (DOING BUSINESS AS NICOR GAS COMPANY)
- --------------------------------------------------------------------------------
               (Exact name of registrant as specified in charter)
 
<TABLE>
<S>                                                <C>
               ILLINOIS                                          36-2863847
       (State of Incorporation)                       (IRS Employer Identification No.)
</TABLE>
 
                                1844 FERRY ROAD
                        NAPERVILLE, ILLINOIS 60563-9600
                        TELEPHONE NUMBER -- 630/983-8888
   (Address and telephone number of registrant's principal executive offices)
 
                               DONALD W. LOHRENTZ
                          VICE PRESIDENT AND TREASURER
                                1844 FERRY ROAD
                        NAPERVILLE, ILLINOIS 60563-9600
                        TELEPHONE NUMBER -- 630/983-8888
           (Name, address and telephone number of agent for service)
 
    Approximate date of commencement of proposed sale to the public: From time
to time after the effective date of the registration statement when warranted by
market conditions and other factors.
 
    If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box. [ ]
 
    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, other than securities offered only in connection with dividend or interest
reinvestment plans, 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, please 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 delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. [ ]
 
                        CALCULATION OF REGISTRATION FEE
 
<TABLE>
<CAPTION>
==========================================================================================================================
                                                               PROPOSED              PROPOSED
                                                               MAXIMUM               MAXIMUM              AMOUNT OF
      TITLE OF EACH CLASS OF           AMOUNT BEING         OFFERING PRICE          AGGREGATE            REGISTRATION
   SECURITIES BEING REGISTERED          REGISTERED            PER UNIT*          OFFERING PRICE*             FEE
- --------------------------------------------------------------------------------------------------------------------------
<S>                                <C>                  <C>                    <C>                  <C>
First Mortgage Bonds                   $175,000,000              100%              $175,000,000            $51,625
==========================================================================================================================
</TABLE>
 
*Estimated solely for purpose of calculating amount of registration fee.
                            ------------------------
 
    THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF
THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A),
MAY DETERMINE.
================================================================================
<PAGE>   2
 
     INFORMATION CONTAINED HEREIN IS SUBJECT TO COMPLETION OR AMENDMENT. A
     REGISTRATION STATEMENT RELATING TO THESE SECURITIES HAS BEEN FILED WITH THE
     SECURITIES AND EXCHANGE COMMISSION. THESE SECURITIES MAY NOT BE SOLD NOR
     MAY OFFERS TO BUY BE ACCEPTED PRIOR TO THE TIME THE REGISTRATION STATEMENT
     BECOMES EFFECTIVE. THIS PROSPECTUS SHALL NOT CONSTITUTE AN OFFER TO SELL OR
     THE SOLICITATION OF AN OFFER TO BUY NOR SHALL THERE BE ANY SALE OF THESE
     SECURITIES IN ANY STATE IN WHICH SUCH OFFER, SOLICITATION OR SALE WOULD BE
     UNLAWFUL PRIOR TO REGISTRATION OR QUALIFICATION UNDER THE SECURITIES LAWS
     OF ANY SUCH STATE.
 
                 SUBJECT TO COMPLETION, DATED DECEMBER 18, 1997
 
                         NORTHERN ILLINOIS GAS COMPANY
                     (DOING BUSINESS AS NICOR GAS COMPANY)
                       $175,000,000 FIRST MORTGAGE BONDS
 
                             ----------------------
 
     Northern Illinois Gas Company, an Illinois corporation doing business as
Nicor Gas Company ("Nicor Gas Company" or the "Company") may offer and sell, in
one or more series, up to $175,000,000 aggregate principal amount of First
Mortgage Bonds (the "New Bonds") on terms determined at the time or times of
sale. The specific aggregate principal amount of any series of New Bonds, public
offering and purchase price, maturity, rate of interest, time of payment of
interest, any redemption terms or other specific terms, and information
concerning distribution (including the names of any underwriters, dealers or
agents, their compensation and the resulting net proceeds to the Company) of the
New Bonds in respect of which this Prospectus is being delivered will be set
forth in a Prospectus Supplement. No sinking fund is to be provided for the New
Bonds.
 
     The New Bonds will be sold directly by the Company or to underwriters or
dealers or through agents named herein (or designated from time to time) or to a
group of underwriters or dealers which may be represented by any one or more of
such firms. (See "Plan of Distribution.")
 
                             ----------------------
 
  THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
 EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS THE SECURITIES
   AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
                               CRIMINAL OFFENSE.
 
                             ----------------------
 
               The date of this Prospectus is December   , 1997.
<PAGE>   3
 
                             AVAILABLE INFORMATION
 
     The Company is subject to the informational requirements of the Securities
Exchange Act of 1934 (the "Exchange Act") and, in accordance therewith, files
reports and other information with the Securities and Exchange Commission (the
"Commission").
 
     Such reports and other information may be inspected and copied at the
offices of the Commission at 450 Fifth Street, N.W., Washington, D.C.; 500 West
Madison Street, Chicago, Illinois; and Seven World Trade Center, New York, New
York, and copies of such material may be obtained from the Public Reference
Section of the Commission in Washington, D.C. 20549 at prescribed rates. The
Company files its reports and other information electronically with the
Commission and these documents can be accessed from the Commission's web site
(http://www.sec.gov).
 
                           INCORPORATION BY REFERENCE
 
     The following documents, which have heretofore been filed by the Company
with the Commission pursuant to the Exchange Act, are incorporated by reference
in this Prospectus and shall be deemed to be a part hereof:
 
          (a) Annual Report on Form 10-K for the year ended December 31, 1996;
     and
 
            (b) Quarterly Reports on Form 10-Q for the quarters ended March 31,
            June 30, and September 30, 1997.
 
     All documents filed by the Company with the Commission pursuant to Section
13(a), 13(c), 14 or 15(d) of the Exchange Act subsequent to the date of this
Prospectus and prior to the termination of the offering made by this Prospectus
shall be deemed to be incorporated herein by reference and to be part hereof
from the date of filing such documents.
 
     The Company hereby undertakes to provide without charge to each person,
including any beneficial owner, to whom a copy of this Prospectus has been
delivered, on the written or oral request of any such person, a copy of any and
all of the documents referred to above which have been or may be incorporated in
this Prospectus by reference, other than exhibits to such documents, unless such
exhibits are specifically incorporated by reference in the information that the
Prospectus incorporates. Written requests should be directed to Donald W.
Lohrentz, Vice President and Treasurer, Nicor Gas Company, P.O. Box 190, Aurora,
Illinois 60507-0190. The Company's telephone number is 630/983-8888.
 
                                  THE COMPANY
 
     Nicor Gas Company is a public utility engaged principally in the purchase,
storage, distribution, sale and transportation of natural gas to the public in
northern Illinois, generally outside the City of Chicago. Nicor Gas Company is
subject to the jurisdiction of the Illinois Commerce Commission ("Ill.C.C.")
which has authority to regulate substantially all phases of the public utility
business in Illinois, including the issuance of long-term debt by the Company.
The Company is a wholly owned subsidiary of Nicor Inc. ("Nicor"), a holding
company exempt from regulation under the Public Utility Holding Company Act of
1935. The New Bonds are not obligations of, or guaranteed by, Nicor. The mailing
address of Nicor Gas Company's general office is P.O. Box 190, Aurora, Illinois
60507-0190. The telephone number is 630/983-8888.
 
                                USE OF PROCEEDS
 
     The net proceeds from the sale of the New Bonds will be used for the
refinancing, if desirable, of certain outstanding First Mortgage Bonds, for
construction programs to the extent not provided by internally generated funds,
and for general corporate purposes.
 
                                        2
<PAGE>   4
 
                              DESCRIPTION OF BONDS
 
     All references to the Company under "Description of Bonds" exclude its
subsidiary. The properties of the Company's subsidiary, which are not material
in the aggregate, are not subject to the lien of the Indenture and do not
constitute bondable property under the Indenture.
 
     GENERAL. The New Bonds will be issued in one or more series under the
Company's Indenture, dated as of January 1, 1954 (as supplemented by
Supplemental Indentures heretofore executed), and Supplemental Indentures (each
of which is hereafter referred to as the "New Supplemental Indenture"), creating
the series in which the New Bonds are to be issued. These series will constitute
the forty-first and subsequent series of bonds issued under the Indenture. The
earlier series of bonds issued under the Indenture that are currently
outstanding are as follows:
 
<TABLE>
<CAPTION>
                                                                AMOUNT AUTHORIZED,
                                                              ISSUED AND OUTSTANDING
SERIES                                                          NOVEMBER 30, 1997
- ------                                                        ----------------------
<S>                                                           <C>
5 7/8% due February 1, 1998.................................       $ 25,000,000
6 1/4% due February 1, 1999.................................         25,000,000
5 7/8% due May 1, 2000......................................         50,000,000
6.45% due August 1, 2001....................................         75,000,000
6.75% due June 1, 2002......................................         50,000,000
8 7/8% due August 15, 2021..................................         50,000,000
8 1/4% due July 15, 2022....................................         75,000,000
7 3/8% due July 1, 2023.....................................         50,000,000
8 1/4% due August 15, 2024..................................         50,000,000
7.26% due October 15, 2025..................................         50,000,000
7 3/8% due October 15, 2027.................................         50,000,000
                                                                   ------------
                                                                   $550,000,000
                                                                   ============
</TABLE>
 
     Each series of the New Bonds will be dated the date of the applicable New
Supplemental Indenture, will mature on such date or dates as are set forth in
such New Supplemental Indenture, and will bear interest at the rate per annum
specified in its title, payable semiannually in each year on the date which
corresponds to the date of the New Supplemental Indenture and on the date which
is six months thereafter, commencing with the date which is six months after the
date of the New Supplemental Indenture. So long as there is no existing default
in the payment of interest on the series of New Bonds, such interest shall be
payable to the person in whose name each bond is registered on the record dates
prescribed by the New Supplemental Indenture (whether or not a business day)
next preceding the respective interest payment dates; provided, however, if and
to the extent that the Company shall default in the payment of interest due on
any such interest payment date, such defaulted interest shall be paid to the
person in whose name each such bond is registered on the record date fixed in
advance by the Company for the payment of such defaulted interest. Both
principal and interest will be payable at the office or agency of the Company in
The City of Chicago or, at the option of the registered owner, at the office or
agency of the Company in The City of New York. Any installment of interest on
the bonds may, at the Company's option, be paid by mailing checks for such
interest payable to or upon the written order of the person entitled thereto to
the address of such person as it appears on the registration books.
 
     The New Bonds will be delivered, in definitive form, as registered bonds in
denominations of $1,000 each or of any authorized multiple thereof. No service
charge will be made for any registration, transfer or exchange of bonds.
 
     Copies of the Indenture and a copy of the applicable New Supplemental
Indenture, in substantially the form in which it is to be executed, except for
certain terms to be set forth in the Prospectus Supplement which will be
inserted after the sale of the series of New Bonds, are exhibits to the
Registration Statement. Reference is made to such exhibits for a complete
statement of the provisions of the Indenture and such New Supplemental
Indenture. The following statements are brief summaries of certain of such
provisions. The
 
                                        3
<PAGE>   5
 
terms "permitted liens," "prior liens," "prior lien bonds," "retired" as used
with respect to bonds, "property additions," "property of the character of
property additions," "mortgage date of acquisition" and "utilized under the
Indenture" are hereinafter used with the meanings ascribed to such terms,
respectively, by the Indenture.
 
     REDEMPTION AND SINKING FUND. Any redemption provisions will be set forth in
the Prospectus Supplement. No sinking fund is to be provided for the New Bonds.
 
     SECURITY. The New Bonds will rank equally and ratably as obligations of the
Company with all bonds, irrespective of series, at any time outstanding under
the Indenture. The Indenture constitutes a direct first mortgage lien on
substantially all gas property and franchises (other than expressly excepted
property) now owned by the Company, subject only to permitted liens. All gas
property and franchises (other than expressly excepted property) hereafter
acquired by the Company will, upon recordation or registration by the Company in
accordance with its agreements under the Indenture, become subject to the lien
of the Indenture, subject only to permitted liens and liens, if any, existing or
placed on such property at the time of acquisition thereof. The Indenture
contains limitations upon the acquisition of property subject to a prior lien
and the transfer of all or substantially all of the property of the Company to
another corporation the property of which is subject to a prior lien. In
general, such limitations are based upon ratios of (a) principal amount of prior
lien bonds to fair value of property securing such bonds and (b) net earnings of
such property to interest charges on such bonds. The Company has agreed in the
Indenture to maintain and protect the mortgaged property as an operating system.
 
     There are expressly excepted from the lien of the Indenture, whether now
owned by the Company or hereafter acquired, certain real estate not used in the
gas utility business, real estate held by the Company in the name of a nominee,
cash and securities not specifically pledged under the Indenture, receivables,
contracts (other than leases), materials, supplies, all gas in storage not
included in the Company's utility plant accounts, merchandise, automobiles,
trucks and other transportation equipment, office furniture and equipment,
natural gas wells, natural gas leases and natural gas gathering lines.
 
     ISSUANCE OF ADDITIONAL BONDS. No bonds of any of the presently outstanding
series may be issued in addition to the bonds of such series now outstanding,
nor will any New Bonds be issuable in addition to the $175,000,000 principal
amount of the New Bonds covered by this Prospectus. Subject to the provisions of
the Indenture, additional bonds of any other series may be issued in principal
amount equal to:
 
          (a) 66 2/3% of "net property additions" not previously utilized under
     the Indenture;
 
          (b) the amount of cash deposited with the Trustee as a basis for the
     issuance of such bonds; and
 
          (c) the amount of "bondable bond retirements" not previously utilized
     under the Indenture, such term being defined, in general, to include (1)
     the principal amount of bonds of any series retired otherwise than through
     a sinking fund, and (2) such percentage, if any, of the principal amount of
     bonds of any other series retired through a sinking fund as may be
     specified in the supplemental indenture creating such series (does not
     include the New Bonds or any previously issued series outstanding);
 
provided, however, that no such bonds in any event may be issued under (a) or
(b), or under (c) if the bonds to be issued bear a higher rate of interest than
that borne by the bonds retired or being retired (except in case such bonds
retired or being retired mature within two years), unless the "net earnings" of
the Company for a 12 month period within the immediately preceding 15 month
period shall have been equal to at least two and one-half times the annual
interest on all bonds then outstanding under the Indenture, including the bonds
then applied for but not including any bonds then being retired, and on all
prior lien bonds then outstanding, not including any prior lien bonds then being
retired.
 
     The term "net earnings" shall mean the earnings of the Company (including
earnings or loss of any gas utility business acquired during the period for that
part of the period prior to acquisition) computed by deducting from the revenues
of the Company the operating expenses, depreciation and taxes of the Company
except (a) charges for the amortization, write-down or write-off of acquisition
adjustments or intangibles; (b) property losses charged to operations; (c)
provisions for income and excess or other profits taxes imposed
 
                                        4
<PAGE>   6
 
on income after the deduction of interest charges, or charges made in lieu of
such taxes; (d) interest charges; and (e) amortization of debt and stock
discount and expense or premium. Any net profit or net loss from merchandising
and jobbing is to be deducted from operating expenses or added to operating
expenses, as the case may be. Net non-operating income from property and
securities not subject to the lien of the Indenture may be included in revenues
but only to the extent of not more than 10% of the total of such net earnings.
No profits or losses on disposition of property or securities or on the
reacquisition of securities shall be included in net earnings.
 
     "Net property additions" means the amount of $9,000,000, plus the cost or
fair value as of the mortgage date of acquisition thereof, whichever is less, of
property additions (which may not include additions subject to a prior lien)
after January 31, 1954, less all "current provisions for depreciation" made
subsequent to that date, after deducting from such current provisions for
depreciation the amount of the "renewal fund requirement," if any, for the year
1954 (last 11 months) and subsequent years.
 
     "Current provisions for depreciation" for any period means the greater of:
 
          (a) the total of the amounts appropriated for depreciation during such
     period on all property of the character of property additions subject to
     the lien of the Indenture but not subject to a prior lien, increased or
     decreased, as the case may be, by net salvage for such period, such amounts
     not to include, however, provisions for depreciation charged to surplus,
     charges to income or surplus for the amortization, write-down or write-off
     of acquisition adjustments or intangibles, property losses charged to
     operations or surplus, or charges to income in lieu of income and excess or
     other profits taxes; or
 
          (b) an amount equal to one-twelfth of 2% for each calendar month of
     such period (or such lesser percentage as may, at intervals of not less
     than five years, be certified by an independent engineer as adequate) of
     the original cost, as of the beginning of such month, of all depreciable
     property of the character of property additions subject to the lien of the
     Indenture but not subject to a prior lien.
 
     As of November 30, 1997, net property additions available as the basis for
the issuance of additional bonds amounted to approximately $431,939,000. As of
November 30, 1997, there were no bondable bond retirements available as the
basis for the issuance of additional bonds. The New Bonds will be issued first
against any newly available bondable bond retirements and the balance against
net property additions.
 
     Other than the security afforded by the lien of the Indenture and the
restrictions thereunder as to issuance of additional first mortgage bonds
described above, there are no provisions in the Indenture which afford holders
of New Bonds protection in the event of a highly leveraged transaction involving
the Company.
 
     RENEWAL FUND REQUIREMENT. Under the Indenture, the Company is required to
pay to the Trustee in each year an amount of cash, as a renewal fund, equal to
the excess, if any, of current provisions for depreciation for such year over
the cost or fair value as of the mortgage date of acquisition thereof, whichever
is less, of property additions for such year. Such amount, which will be the
renewal fund requirement for such year, is subject to reduction by an amount
equal to the amount, certified to the Trustee, of net property additions or
bondable bond retirements, or both, not previously utilized under the Indenture.
There was no renewal fund requirement for any of the years 1954 through 1995.
The amount of the renewal fund requirement for the year 1996 was approximately
$6,658,000, which amount was reduced by the utilization of an equivalent amount
of net property additions.
 
     RELEASE OF PROPERTY AND WITHDRAWAL CASH. The Company may sell or otherwise
dispose of and secure the release of any part of the mortgaged property upon
deposit with the Trustee of cash in an amount equal to the fair value of the
property released, which in no event may be less than the consideration
received. If, however, the property to be released is subject to a prior lien,
such property may be released upon a showing that it has been released from such
prior lien upon due compliance with the release provisions thereof. Cash to be
deposited upon the release of mortgaged property may, subject to certain
limitations, be reduced by an amount not in excess of (a) the principal amount
of purchase money obligations secured by purchase money mortgage upon the
property to be released, and (b) the principal amount of any prior lien bonds
secured by a prior lien on the property to be released, the payment of which
bonds has been assumed by the purchaser.
 
                                        5
<PAGE>   7
 
Such cash may also be reduced by an amount equal to the amount of net property
additions or bondable bond retirements, or both, not previously utilized under
the Indenture.
 
     Cash deposited with the Trustee as a basis for the issuance of bonds may be
withdrawn in an amount equal to 66 2/3% of the amount of net property additions
not previously utilized under the Indenture, or in an amount equal to the amount
of bondable bond retirements not previously utilized under the Indenture, or
both. All other cash (except sinking fund cash) deposited with the Trustee to be
held as part of the mortgaged property may be withdrawn in an amount equal to
the amount of net property additions or bondable bond retirements, or both, not
previously utilized under the Indenture.
 
     MODIFICATION OF INDENTURE. In general, modifications or alterations of the
Indenture and indentures supplemental thereto and of the rights and obligations
of the Company and of the bondholders, and waivers of compliance with the
Indenture and indentures supplemental thereto, may with the approval of the
Company be made upon the affirmative vote of holders of 66 2/3% in principal
amount of the bonds entitled to vote with respect to the matter involved, but no
such modifications or alterations or waivers of compliance are permitted with
respect to certain basic matters, such as terms of payment of principal,
premium, if any, and interest on bonds (but not including sinking fund
requirements) and, otherwise than as permitted by the Indenture, creation of
liens ranking prior to or on a parity with the lien of the Indenture with
respect to any of the mortgage property or the deprivation of any bondholder of
his lien, or any reduction of the vote necessary to take such actions.
 
     DEFAULTS. The following are deemed as "completed defaults": default in the
payment of principal on any bonds issued under the Indenture, including the New
Bonds, when due and payable at maturity or upon redemption or by declaration or
as otherwise provided in the Indenture; default for 60 days in the payment of
interest on any of such bonds; default for 30 days after written notice to the
Company in the payment of principal or interest (after the expiration of any
applicable grace period) on prior lien bonds; default for 90 days after written
notice to the Company in the performance of any other covenant of the Indenture,
including any covenant relating to sinking fund obligations; and in certain
events of bankruptcy and insolvency. The Company furnishes annually to the
Trustee a certificate in respect of compliance or noncompliance by the Company
with the covenants of the Indenture.
 
     In case one or more completed defaults shall occur and be continuing, the
Trustee or the holders of not less than 25% in principal amount of the bonds
then outstanding may declare the principal of all such bonds, together with all
accrued and unpaid interest thereon, if not already due, to be due and payable
immediately. No bondholder has the right to institute any proceeding for the
enforcement of the Indenture unless such holder shall have given the Trustee
written notice of some existing default and continuance thereof and the holders
of not less than 25% in principal amount of the bonds then outstanding shall
have requested the Trustee in writing to take action and have given the Trustee
a reasonable opportunity to take such action. The Trustee is entitled to be
indemnified before taking action to enforce the lien at the request of such
bondholders.
 
     If, at any time after the occurrence of a completed default and before the
sale of the mortgage property shall have been made, all arrears of principal and
interest, together with other amounts described in the Indenture, shall either
be paid by the Company or be collected and paid out of the mortgage property and
all other defaults, if any, which shall have occurred, shall be remedied or
cured, to the reasonable satisfaction of the Trustee, then the Trustee, upon the
written request of the holders of a majority in principal amount of the bonds
then outstanding, shall waive any such default.
 
     CONCERNING THE TRUSTEE. Harris Trust and Savings Bank ("Harris") is the
Trustee under the Indenture. The Company and certain of its affiliates borrow
from, and utilize many of the banking services offered by, Harris in the normal
course of business.
 
                                        6
<PAGE>   8
 
                              PLAN OF DISTRIBUTION
 
     Nicor Gas Company may sell the New Bonds, in one or more series, for
reoffering at fixed or varying prices to the public through underwriting
syndicates led by one or more managing underwriters or through one or more of
such firms acting alone. Firms which may act as managing underwriters with
respect to the New Bonds include Credit Suisse First Boston Corporation, Merrill
Lynch, Pierce, Fenner & Smith Incorporated, Morgan Stanley & Co. Incorporated,
PaineWebber Incorporated, Prudential Securities Incorporated and Salomon
Brothers Inc. The specific managing underwriter or underwriters for each series
will be set forth in the Prospectus Supplement relating to the specific series
of New Bonds, together with the members of the underwriting syndicate, if any.
If an underwriter is utilized in the sale, Nicor Gas Company will execute an
underwriting agreement providing that the obligations of the underwriters are
subject to certain conditions precedent and that the underwriters will be
obligated to purchase all the New Bonds of the series being sold if any are
purchased.
 
     The New Bonds may also be sold directly by Nicor Gas Company or through
agents designated by the Company from time to time. Any agent involved in the
offer or sale of the New Bonds of any series will be named in the Prospectus
Supplement relating thereto. Unless otherwise indicated in the Prospectus
Supplement, any such agent will act on a best efforts basis for the period of
its appointment.
 
     In connection with the sale of any series of the New Bonds, underwriters or
agents may receive compensation from the Company in the form of discounts,
concessions or commissions. Underwriters, agents or dealers that participate in
the distribution of any series of the New Bonds may be deemed to be underwriters
of such series, and any discounts or commissions received by them from Nicor Gas
Company and any profit on the resale of the New Bonds of such series by them may
be deemed to be underwriting discounts and commissions under the Securities Act
of 1933, as amended (the "Act"). The Prospectus Supplement will describe such
discounts and commissions, all other underwriting compensation, and discounts
and concessions to be allowed or paid to dealers. Underwriters or agents may be
entitled under agreements entered into with Nicor Gas Company to indemnification
by Nicor Gas Company against certain civil liabilities, including liabilities
under the Act. Underwriters or agents may be customers of, engage in
transactions with, or perform services for the Company in the ordinary course of
business.
 
     If a dealer is utilized in the sale of any series of the New Bonds, Nicor
Gas Company will sell the New Bonds of such series to the dealer as principal.
The dealer may then resell the New Bonds of such series to the public at varying
prices to be determined by such dealer at the time of resale. Other sales may be
made, directly or through agents, to purchasers outside existing trading
markets.
 
                                 LEGAL OPINIONS
 
     Certain legal matters in connection with the offering of the New Bonds will
be passed upon for the Company by Mayer, Brown & Platt, 190 South LaSalle
Street, Chicago, Illinois 60603, and by Wildman, Harrold, Allen & Dixon, 225
West Wacker Drive, Chicago, Illinois 60606 for any underwriter, dealer or agent
named in the Prospectus Supplement.
 
                                    EXPERTS
 
     The consolidated financial statements and schedule included in the Annual
Report on Form 10-K for the year ended December 31, 1996, incorporated by
reference in this Registration Statement, have been audited by Arthur Andersen
LLP, independent public accountants, as indicated in their report with respect
thereto, and are included herein in reliance upon the authority of said firm as
experts in giving said report.
 
                                        7
<PAGE>   9
 
=======================================================
 
NO DEALER, SALESMAN OR OTHER PERSON HAS BEEN AUTHORIZED TO GIVE ANY INFORMATION
OR TO MAKE ANY REPRESENTATION NOT CONTAINED IN THIS PROSPECTUS AND, IF GIVEN OR
MADE, SUCH INFORMATION OR REPRESENTATION MUST NOT BE RELIED UPON AS HAVING BEEN
AUTHORIZED BY THE COMPANY OR ANY UNDERWRITER. THIS PROSPECTUS DOES NOT
CONSTITUTE AN OFFER TO SELL OR A SOLICITATION OF ANY OFFER TO BUY ANY OF THE
SECURITIES OFFERED HEREBY IN ANY JURISDICTION TO ANY PERSON TO WHOM IT IS
UNLAWFUL TO MAKE SUCH OFFER IN SUCH JURISDICTION. NEITHER THE DELIVERY OF THIS
PROSPECTUS NOR ANY SALE MADE HEREUNDER SHALL, UNDER ANY CIRCUMSTANCES, CREATE
ANY IMPLICATION THAT THERE HAS BEEN NO CHANGE IN THE AFFAIRS OF THE COMPANY
SINCE THE DATE HEREOF.
 
                            ------------------------
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                         PAGE
                                         ----
<S>                                      <C>
Available Information..................    2
Incorporation by Reference.............    2
The Company............................    2
Use of Proceeds........................    2
Description of Bonds...................    3
Plan of Distribution...................    7
Legal Opinions.........................    7
Experts................................    7
</TABLE>
 
=======================================================
=======================================================
 
                                 [Company Logo]
                                 $     ,000,000
 
                              FIRST MORTGAGE BONDS
 
                                        % SERIES
 
                             DUE                  ,
 
                                   PROSPECTUS
 
=======================================================
<PAGE>   10
 
                                    PART II
 
                   INFORMATION NOT REQUIRED IN THE PROSPECTUS
 
          THE TERM "COMPANY" REFERS TO NORTHERN ILLINOIS GAS COMPANY.
 
ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.
 
     Estimated expenses of the Company in connection with the issuance and
distribution of the New Bonds:
 
<TABLE>
<S>                                                             <C>
Bond rating fees............................................    $275,000
Trustee's fees and expenses.................................      20,000
Printing....................................................      50,000
Registration fee -- Securities and Exchange Commission......      51,625
Legal fees..................................................     133,000
"Blue Sky" fees and expenses................................      20,000
Preparation and delivery of New Bonds.......................      10,000
Accountants' fees and expenses..............................      30,000
Miscellaneous...............................................      30,375
                                                                --------
          Total.............................................    $620,000
                                                                ========
</TABLE>
 
ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS.
 
     The Company is incorporated in Illinois. Section 8.75 of the Illinois
Business Corporation Act of 1983 permits, and in some circumstances requires,
indemnification of officers, directors and employees of the Company.
 
     The Articles of Incorporation of the Company provide indemnification for
all officers, directors or employees of the Company. Coverage is also extended
to persons who, at the request of the Company, serve, in any capacity, other
corporations, associations or entities. Indemnification is provided, except in
relation to matters as to which it is finally adjudged or determined that the
person breached a duty to the Company, or the person failed to act in good faith
for a purpose which the person reasonably believed to be in the best interest of
the Company, or, in the case of criminal litigation, the person had reasonable
cause to believe that such conduct was unlawful. If the required indemnification
standard is met, indemnification may cover the liabilities and reasonable
expenses of the person.
 
     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
Company pursuant to the foregoing provisions, or otherwise, the Company has been
advised that in the opinion of the Securities and Exchange Commission such
indemnification is against public policy as expressed in the Act and is,
therefore, unenforceable. In the event that a claim for indemnification against
such liabilities (other than the payment by the Company of expenses incurred or
paid by a director, officer or controlling person of the Company in the
successful defense of any action, suit or proceeding, or claims to the extent
covered by contracts of insurance) is asserted by such director, officer or
controlling person in connection with the securities being registered, the
Company will, unless in the opinion of its counsel the matter has been settled
by controlling precedent, submit to a court of appropriate jurisdiction the
question of whether such indemnification by it is against public policy as
expressed in the Act and will be governed by the final adjudication of such
issue.
 
     The Company maintains at its expense insurance policies which insure the
Company and the officers and directors of the Company against certain
liabilities, including certain liabilities which might arise under the
Securities Act of 1933.
 
ITEM 16. EXHIBITS.
 
     See Exhibit Index, page S-4.
 
                                       S-1
<PAGE>   11
 
ITEM 17. UNDERTAKINGS.
 
                               RULE 415 OFFERING
 
     The undersigned registrant hereby undertakes:
 
          (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;
 
             (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. 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 a prospectus filed with the Commission pursuant
        to Rule 424(b) if, in the aggregate, the changes in volume and price
        represent no more than a 20% change in the maximum aggregate offering
        price set forth in the "Calculation of Registration Fee" table in the
        effective registration statement.
 
     Provided, however, that paragraphs (1)(i) and (1)(ii) do not apply if the
     registration statement is on Form S-3 or Form S-8 and the information
     required to be included in a post-effective amendment by those paragraphs
     is contained in periodic reports filed with the Commission by the
     registrant pursuant to section 13 or section 15(d) of the Securities
     Exchange Act of 1934 that are incorporated by reference 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.
 
          (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.
 
      FILINGS INCORPORATING SUBSEQUENT EXCHANGE ACT DOCUMENTS BY REFERENCE
 
     The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to section 13(a) or section 15(d) of the
Securities Exchange Act of 1934 that is incorporated by reference in the
registration statement 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.
 
                         ACCELERATION OF EFFECTIVENESS
 
     Reference is made to Item 15, Indemnification of Directors and Officers,
for this Undertaking.
 
                                       S-2
<PAGE>   12
 
                                   SIGNATURES
 
     Pursuant to the requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and that the security rating requirement
specified in Transaction Requirement B.2. of Form S-3 will be met by the time of
effectiveness and has duly caused this Registration Statement or Amendment
thereto to be signed on its behalf by the undersigned, thereunto duly
authorized, at its general office, 1844 Ferry Road, Naperville, Illinois, on the
18th day of December 1997.
 
                                          NORTHERN ILLINOIS GAS COMPANY
                                          (d/b/a Nicor Gas Company)
 
                                          By          DAVID L. CYRANOSKI
                                            ------------------------------------
                                                     David L. Cyranoski
                                                   Senior Vice President,
                                                  Secretary and Controller
 
     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement or Amendment thereto has been signed by the following
persons in the capacities indicated on December 18, 1997.
 
<TABLE>
<CAPTION>
                  SIGNATURE                                        TITLE
                  ---------                                        -----
<S>                                            <C>
 
THOMAS L. FISHER                               Chairman, President, Chief Executive Officer
- ---------------------------------------------  and Director
Thomas L. Fisher
 
DAVID L. CYRANOSKI                             Senior Vice President, Secretary and
- ---------------------------------------------  Controller and Principal Financial and
David L. Cyranoski                             Accounting Officer
 
ROBERT M. BEAVERS, JR.*                        Director
 
BRUCE P. BICKNER*                              Director
 
JOHN H. BIRDSALL, III*                         Director
 
W. H. CLARK                                    Director
 
JOHN E. JONES*                                 Director
 
DENNIS J. KELLER*                              Director
 
CHARLES S. LOCKE*                              Director
 
SIDNEY R. PETERSEN*                            Director
 
DANIEL R. TOLL*                                Director
 
PATRICIA A. WIER*                              Director
</TABLE>
 
                                          *By          GEORGE M. BEHRENS
                                             -----------------------------------
                                             George M. Behrens
                                              (Attorney-in-fact)
 
                                       S-3
<PAGE>   13
 
                                 EXHIBIT INDEX
 
                            EXHIBITS FILED HEREWITH
 
<TABLE>
<CAPTION>
EXHIBIT
NUMBER                       DESCRIPTION OF DOCUMENT
- -------                      -----------------------
<C>        <S>
 1.01      Form of proposed Underwriting Agreement.
 4.20      Form of proposed Supplemental Indenture.
 5.01      Legal Opinion.
23.01      Consent of Independent Public Accountants.
23.02      Consent of Counsel. (Reference is made to Exhibit 5.01 for
           this Consent.)
24.01      Powers of Attorney.
25.01      Statement of Eligibility of Trustee (Form T-1).
</TABLE>
 
                       EXHIBITS INCORPORATED BY REFERENCE
 
     The exhibits listed below have been heretofore filed with the Securities
and Exchange Commission as exhibits to registration statements or to other
filings with the Commission and are incorporated herein as exhibits by
reference. The file number and exhibit number of each such exhibit are stated,
in parentheses, in the description of such exhibit.
 
<TABLE>
<CAPTION>
EXHIBIT
NUMBER                       DESCRIPTION OF DOCUMENT
- -------                      -----------------------
<C>        <S>
 4.01      Indenture of Commonwealth Edison Company to Continental
           Illinois National Bank and Trust Company of Chicago,
           Trustee, dated as of January 1, 1954. (File No. 1-7296, Form
           10-K for 1995, Exhibit 4.01.)
 4.02      Indenture of Adoption of the Company to Continental Illinois
           National Bank and Trust Company of Chicago, Trustee, dated
           February 9, 1954. (File No. 1-7296, Form 10-K for 1995,
           Exhibit 4.02.)
 4.03      Supplemental Indenture, dated June 1, 1963, of the Company
           to Continental Illinois National Bank and Trust Company of
           Chicago, Trustee, under Indenture dated as of January 1,
           1954. (File No. 2-21490, Form S-9, Exhibit 2-8.)
 4.04      Supplemental Indenture, dated May 1, 1966, of the Company to
           Continental Illinois National Bank and Trust Company of
           Chicago, Trustee, under Indenture dated as of January 1,
           1954. (File No. 2-25292, Form S-9, Exhibit 2-4.)
 4.05      Supplemental Indenture, dated June 1, 1971, of the Company
           to Continental Illinois National Bank and Trust Company of
           Chicago, Trustee, under Indenture dated as of January 1,
           1954. (File No. 2-44647, Form S-7, Exhibit 2-03.)
 4.06      Supplemental Indenture, dated April 30, 1976, between Nicor
           Inc. and Continental Illinois National Bank and Trust
           Company of Chicago, Trustee, under Indenture dated as of
           January 1, 1954. (File No. 2-56578, Form S-9, Exhibit 2-25.)
 4.07      Supplemental Indenture, dated April 30, 1976, of the Company
           to Continental Illinois National Bank and Trust Company of
           Chicago, Trustee, under Indenture dated as of January 1,
           1954. (File No. 2-56578, Form S-9, Exhibit 2-21.)
 4.08      Supplemental Indenture, dated July 1, 1989, of the Company
           to Continental Bank, National Association, Trustee, under
           Indenture dated as of January 1, 1954. (File No. 1-7296,
           Form 8-K for June 1989, Exhibit 4-01.)
</TABLE>
 
                                       S-4
<PAGE>   14
<TABLE>
<CAPTION>
EXHIBIT
NUMBER                       DESCRIPTION OF DOCUMENT
- -------                      -----------------------
<C>        <S>
 4.09      Supplemental Indenture, dated August 15, 1991, of the
           Company to Continental Bank, National Association, Trustee,
           under Indenture dated as of January 1, 1954. (File No.
           1-7296, Form 8-K for August 1991, Exhibit 4-01.)
 4.10      Supplemental Indenture, dated July 15, 1992, of the Company
           to Continental Bank, National Association, Trustee, under
           Indenture dated as of January 1, 1954. (File No. 1-7296,
           Form 10-Q for June 1992, Exhibit 4-01.)
 4.11      Supplemental Indenture, dated February 1, 1993, of the
           Company to Continental Bank, National Association, Trustee,
           under Indenture dated as of January 1, 1954. (File No.
           1-7296, Form 10-K for 1992, Exhibit 4-17.)
 4.12      Supplemental Indenture, dated May 1, 1993, of the Company to
           Continental Bank, National Association, Trustee, under
           Indenture dated as of January 1, 1954. (File No. 1-7296,
           Form 10-Q for March 1993, Exhibit 4-02.)
 4.13      Supplemental Indenture, dated July 1, 1993, of the Company
           to Continental Bank, National Association, Trustee, under
           Indenture dated as of January 1, 1954. (File No. 1-7296,
           Form 10-Q for June 1993, Exhibit 4-01.)
 4.14      Supplemental Indenture, dated August 15, 1994, of the
           Company to Continental Bank, Trustee, under Indenture dated
           as of January 1, 1954. (File No. 1-7296, Form 10-Q for
           September 1994, Exhibit 4.01.)
 4.15      Supplemental Indenture, dated October 15, 1995, of the
           Company to Bank of America Illinois, Trustee, under
           Indenture dated as of January 1, 1954. (File No. 1-7296,
           Form 10-Q for September 1995, Exhibit 4.01.)
 4.16      Supplemental Indenture, dated May 10, 1996, of the Company
           to Harris Trust and Savings Bank, Trustee, under Indenture
           dated as of January 1, 1954. (File No. 1-7296, Form 10-Q for
           June 1996, Exhibit 4.01.)
 4.17      Supplemental Indenture, dated August 1, 1996, of the Company
           to Harris Trust and Savings Bank, Trustee, under Indenture
           dated as of January 1, 1954. (File No. 1-7296, Form 10-Q for
           June 1996, Exhibit 4.02.)
 4.18      Supplemental Indenture, dated June 1, 1997, of the Company
           to Harris Trust and Savings Bank, Trustee, under Indenture
           dated as of January 1, 1954. (File No. 1-7296, Form 10-Q for
           June 1997, Exhibit 4.01.)
 4.19      Supplemental Indenture, dated October 15, 1997, of the
           Company to Harris Trust and Savings Bank, Trustee, under
           Indenture dated as of January 1, 1954. (File No. 1-7296,
           Form 10-Q for September 1997, Exhibit 4.01.)
12.01      Consolidated Ratio of Earnings to Fixed Charges. (File No.
           1-7296, Form 10-Q for September 1997, Exhibit 12.01)
</TABLE>
 
                                       S-5

<PAGE>   1
 
                                                                    EXHIBIT 1.01
 
                         NORTHERN ILLINOIS GAS COMPANY
 
                                 $    ,000,000
 
                              FIRST MORTGAGE BONDS
 
                                     % SERIES DUE
 
                             UNDERWRITING AGREEMENT
 
[NAME(S) OF REPRESENTATIVES]
As Representative(s) of the several Underwriters
                                                                           , 199
 
Dear Sirs:
 
     Northern Illinois Gas Company, doing business as Nicor Gas Company ("Nicor
Gas" or the "Company") proposes, subject to the terms and conditions stated
herein and in the General Terms and Conditions of Underwriting Agreement in the
form of Annex A hereto, a copy of which you have previously received, to issue
and sell to the Underwriters named in Schedule I hereto (the "Underwriters"),
$   ,000,000 aggregate principal amount of the Company's First Mortgage Bonds
(the "Bonds"). All of the provisions of such General Terms and Conditions of
Underwriting Agreement are incorporated herein by reference in their entirety,
and shall be deemed to be a part of this Underwriting Agreement to the same
extent as if such provisions had been set forth in full herein. Unless otherwise
defined herein, terms defined in the General Terms and Conditions of
Underwriting Agreement are used herein as therein defined.
 
     An amendment to the Registration Statement, or a supplement to the
Prospectus, as the case may be, relating to the Bonds in the form heretofore
delivered to you is now proposed to be filed or mailed for filing with the
Commission. Such amendment or supplement sets forth the terms of the Bonds.
 
     Subject to the terms and conditions set forth herein, the Company agrees to
issue and sell to the Underwriters, and the Underwriters agree, severally and
not jointly, to purchase from the Company, the principal amount of Bonds set
forth opposite the names of such Underwriters in Schedule I hereto on the
following terms and conditions:
 
<TABLE>
<CAPTION>
       <S>                                              <C>
       Aggregate principal amount of Bonds to be        $   ,000,000
         purchased:.................................
       Rate of interest per annum to be borne by the    % (such rate to be a multiple of .001%)
         Bonds (payable semiannually):..............
       Maturity date of the Bonds:..................
       Price to be paid to the Company for the          % of the principal amount of the Bonds (not less
         Bonds:.....................................    than 99%) plus accrued interest from date of
                                                        Supplemental Indenture to the date of delivery
                                                        of the Bonds.
       Initial public offering price of the             % of the principal amount of the Bonds plus
         Bonds:.....................................    accrued interest from date of Supplemental
                                                        Indenture to the date of delivery of the Bonds.
                                                        (If other, give details.)
       Place for delivery of Bonds:.................
       Date and time of Time of Delivery:...........
       Place for checking Bonds on the business day
         prior to Time of Delivery:.................
</TABLE>
 
                                        1
<PAGE>   2
<TABLE>
<CAPTION>
       <S>                                              <C>
       Redemption and Sinking Fund:.................    Any redemption provisions will be as set forth
                                                        on Schedule II hereto. No sinking fund will be
                                                        provided.
       Address for notices per Section 12 of the
         General Terms and Conditions of
         Underwriting Agreement:....................
</TABLE>
 
     If the foregoing is in accordance with your understanding, please sign and
return to us the enclosed counterparts hereof, whereupon it will become a
binding agreement between the Underwriters and the Company in accordance with
its terms. It is understood that your acceptance of this letter on behalf of
each of the Underwriters constitutes your representation that you are authorized
to execute this Underwriting Agreement on behalf of each of the Underwriters.
 
                                          Very truly yours,
 
                                          NORTHERN ILLINOIS GAS COMPANY
 
                                          By
 
                                            ------------------------------------
                                            Vice President and Treasurer
 
The foregoing Underwriting Agreement
     is hereby confirmed and accepted
     as of the date first above written.
 
[NAMES(S) OF REPRESENTATIVES]
 
BY 
   ----------------------------------
 
On behalf of itself and as Representatives(s) of each of the Underwriters named
in Schedule I hereto.
 
                                        2
<PAGE>   3
 
                                                                      SCHEDULE I
 
<TABLE>
<CAPTION>
                                                              PRINCIPAL AMOUNT
                    NAME OF UNDERWRITER                           OF BONDS
                    -------------------                       ----------------
<S>                                                           <C>
</TABLE>
<PAGE>   4
 
                                                                     SCHEDULE II
 
                      (Alternative Redemption Provisions)
 
     [Redemption. The Bonds may not be called for redemption by the Company
prior to             . On                and thereafter until maturity on
               , on not less than 30 days' notice, the Bonds will be redeemable
at the option of the Company, as a whole at any time or in part from time to
time, at the applicable redemption price (expressed as a percentage of principal
amount) set forth below under "General Redemption Prices," plus accrued and
unpaid interest to the redemption date:
 
<TABLE>
<CAPTION>
                    GENERAL REDEMPTION PRICES
- ------------------------------------------------------------------
                  IF REDEEMED DURING 12 MONTHS'
                       PERIOD BEGINNING *:
- ------------------------------------------------------------------
        YEAR            PERCENTAGE        YEAR        PERCENTAGE
        ----            ----------        ----        ----------
<S>                    <C>            <C>            <C>
                       %                             %
 
</TABLE>
 
DETERMINATION OF GENERAL REDEMPTION PRICES
 
     The general redemption prices, expressed as a percentage of principal
amount, shall be calculated as follows: (a) for the 12-month period beginning
the date of the Supplemental Indenture, the initial public offering price, plus
one year's interest; (b) for the 12-month period beginning one year after the
date of the Supplemental Indenture and for each of the 12-month periods
thereafter through the date        years prior to maturity, the price obtained
by deducting from the redemption price during the prior 12-month period, an
amount which will amortize the premium in excess of 100% as specified in (a)
above over the number of 12-month periods between the date of the Supplemental
Indenture and        years prior to maturity; (c) for the      years prior to
maturity, 100% of the principal amount.
 
     The calculations required shall be made on the basis of the interest rate
specified in the Underwriting Agreement. If any redemption price would not be a
multiple of .01%, such price shall be increased to the next higher such
multiple.
- ------------
 * Insert anniversary date of Supplemental Indenture]
 
     [Redemption. The Bonds may not be called for redemption by the Company
prior to        . On             and thereafter until maturity on        , the
Bonds, on not less than 30 days' notice, will be redeemable at the option of the
Company, as a whole at any time or in part from time to time, at 100% of the
principal amount thereof, plus accrued and unpaid interest to the redemption
date.]
<PAGE>   5
 
                                                                         ANNEX A
 
                         NORTHERN ILLINOIS GAS COMPANY
 
                                  $   ,000,000
 
                              FIRST MORTGAGE BONDS
 
             GENERAL TERMS AND CONDITIONS OF UNDERWRITING AGREEMENT
 
     Northern Illinois Gas Company, an Illinois corporation (the "Company"),
proposes to enter into an Underwriting Agreement into which these General Terms
and Conditions are incorporated by reference (the "Underwriting Agreement") and,
subject to the terms and conditions stated therein, to issue and sell to the
underwriter or underwriters named in Schedule I to the Underwriting Agreement up
to $   ,000,000 aggregate principal amount of its First Mortgage Bonds
(hereinafter called the "Bonds") under the registration statement referred to in
Section 2(a) hereof. Such Bonds will be issued under the Company's Indenture
dated as of January 1, 1954, to Continental Bank, National Association, Trustee
(the "Trustee"), as supplemented by supplemental indentures dated February 9,
1954, April 1, 1956, June 1, 1959, July 1, 1960, June 1, 1963, July 1, 1963,
August 1, 1964, August 1, 1965, May 1, 1966, August 1, 1966, July 1, 1967, June
1, 1968, December 1, 1969, August 1, 1970, June 1, 1971, July 1, 1972, July 1,
1973, April 1, 1975, April 30, 1976, April 30, 1976, July 1, 1976, August 1,
1976, December 1, 1977, January 15, 1979, December 1, 1981, March 1, 1983,
October 1, 1984, December 1, 1986, March 15, 1988, July 1, 1988, July 1, 1989,
July 15, 1990, August 15, 1991, July 15, 1992, February 1, 1993, March 15, 1993,
May 1, 1993, July 1, 1993, August 15, 1994, October 15, 1995, May 10, 1996,
August 1, 1996, June 1, 1997 and October 15, 1997 respectively, and as to be
further supplemented by a Supplemental Indenture (the "Supplemental Indenture")
which will be dated the first or fifteenth day of the calendar month in which
the "Time of Delivery" (as hereinafter defined) falls, creating the series in
which the Bonds are to be issued. Said Indenture as so supplemented is
hereinafter called the "Indenture." The term "Underwriters" herein shall refer
to the several persons, firms and corporations named in Schedule I to the
Underwriting Agreement and the term "Representatives" herein shall refer to the
Underwriters identified as the Representatives who are acting on behalf of the
Underwriters (including themselves) in the Underwriting Agreement. All
obligations of the Underwriters under the Underwriting Agreement are several and
not joint. The terms "Underwriters", "Representatives", "persons", "firms" and
"corporations" shall include the singular as well as the plural.
 
     The terms of the issuance of the Bonds shall be as specified in the
Underwriting Agreement. The Underwriting Agreement shall constitute an agreement
by the Company and the Underwriters to be bound by all of the provisions of
these General Terms and Conditions of Underwriting Agreement, as follows:
 
     SECTION 1. Sale of Bonds. Sales of the Bonds will be made to the
Underwriters, for whom the Representatives will act as such. The obligation of
the Company to issue and sell any of the Bonds and the obligation of any of the
Underwriters to purchase any of the Bonds shall be evidenced by the Underwriting
Agreement. The Underwriting Agreement shall specify the aggregate principal
amount of Bonds to be purchased, the rate and time of payment of interest to be
borne by the Bonds, the maturity date of the Bonds, the price to be paid to the
Company for the Bonds, the initial public offering price or other offering terms
of such Bonds and the redemption prices and other special terms, if any,
relating to the Bonds, the names of the Underwriters of such Bonds, the names of
the Representatives of such Underwriters and the amount of Bonds to be purchased
by each Underwriter, and, subject to the provisions of Section 3 hereof, shall
set forth the date, time and manner of the delivery of such Bonds. The terms of
the Bonds will be set forth in the Prospectus Supplement (as hereinafter
defined). The Underwriting Agreement shall be in the form of an executed writing
(which may be in counterparts) and may be evidenced by an exchange of telecopied
communications or any other rapid transmission device to produce a written
record of communications transmitted.
<PAGE>   6
 
     SECTION 2. Representations and Warranties of the Company. The Company
represents and warrants to, and agrees with, the several Underwriters that:
 
          (a) A registration statement on Form S-3 with respect to the Bonds,
     including a related preliminary prospectus, has been prepared by the
     Company in conformity with the requirements of the Securities Act of 1993,
     as amended (the "Act"), and the rules and regulations of the Securities and
     Exchange Commission (the "Commission") under the Act (the "Regulations"),
     and has been filed with the Commission on                     and, if one
     or more amendments to such registration statement, which may include an
     amended preliminary prospectus, have been filed with the Commission, such
     amendments have been similarly prepared; and such registration statement
     has become effective. Such registration statement, as amended to the date
     of the Underwriting Agreement, together with the prospectus supplement
     referred to below is hereinafter referred to as the "Registration
     Statement". Such prospectus as supplemented specifically relating to the
     Bonds and filed with the Commission under Rule 424(b) of the Act is
     hereinafter referred to as the "Prospectus". The Prospectus has been
     prepared by the Company in conformity with the requirements of the Act and
     the Regulations. Copies of the Registration Statement and any related
     prospectus have been delivered to the Representatives. As used herein,
     Registration Statement, Prospectus and preliminary prospectus shall
     include, in each case, the material incorporated therein pursuant to Item
     12 of Form S-3 filed under the Securities Exchange Act of 1934 (the "1934
     Act") on or prior to the date of the Underwriting Agreement, and "amended",
     "amendment" or "supplement" with respect to the Registration Statement or
     the Prospectus shall be deemed to include the filing by the Company of any
     document pursuant to Sections 13(a), 13(c), 14 or 15(d) of the 1934 Act
     after the date of the Underwriting Agreement.
 
          (b) The registration statement at the time it became effective, and
     the related prospectus and any amendments and supplements thereto filed
     prior to the date of the Underwriting Agreement, conformed in all material
     respects to the provisions of the Act and the Trust Indenture Act of 1939,
     as amended (the "Trust Indenture Act") and the rules and regulations of the
     Commission thereunder, on the date of the Underwriting Agreement and at the
     Time of Delivery (referred to in Section 3) the Registration Statement, the
     Prospectus, and any amendments and supplements thereto, and the Indenture,
     will conform in all material respects to the Act, the Trust Indenture Act
     and the respective rules and regulations of the Commission thereunder; and
     at the time the registration statement became effective, the registration
     statement and related prospectus did not contain any untrue statement of a
     material fact or omit to state a material fact required to be stated
     therein or necessary to make the statements therein not misleading; and at
     the date of this Underwriting Agreement and at the Time of Delivery, the
     Registration Statement and the Prospectus and any amendments and
     supplements thereto do not and will not contain any untrue statement of a
     material fact or omit to state a material fact necessary in order to make
     the statements therein not misleading; provided, however, that none of the
     representations and warranties in this subsection shall apply to statements
     in or omissions from the Registration Statement or Prospectus or any
     amendment or supplement thereto made in reliance upon and in conformity
     with information respecting the Underwriters furnished to the Company in
     writing by or on behalf of any Underwriter through the Representatives
     expressly for use in the Registration Statement or Prospectus.
 
          (c) The documents incorporated by reference into the Prospectus, at
     the time they were filed with the Commission, complied in all material
     respects with the requirements of the 1934 Act and the rules and
     regulations of the Commission thereunder (the "1934 Regulations"), and, at
     the date of this Underwriting Agreement and at the Time of Delivery, when
     read together with the Prospectus and any supplement thereto will not
     contain an untrue statement of a material fact or omit to state a material
     fact required to be stated therein or necessary to make the statements
     therein not misleading, and any documents filed after the date of the
     Underwriting Agreement and so incorporated by reference in the Prospectus
     will, when they are filed with the Commission, comply in all material
     respects with the requirements of the 1934 Act and the 1934 Regulations,
     and when read together with the Prospectus and any supplement thereto will
     not contain an untrue statement of material fact or omit to state a
     material fact required to be stated therein or necessary to make the
     statements therein not misleading.
 
                                        2
<PAGE>   7
 
          (d) Arthur Andersen LLP are independent public accountants with
     respect to the Company and its subsidiaries as required by the Act and the
     Regulations.
 
          (e) The financial statements included in the Registration Statement
     present fairly the financial position of the Company and its consolidated
     subsidiaries as of the dates indicated and the results of their operations
     for the periods specified, and said financial statements have been prepared
     in conformity with generally accepted accounting principles applied on a
     consistent basis during the periods involved.
 
          (f) The Company is a corporation in good standing, duly organized and
     validly existing under the laws of Illinois, and has due corporate
     authority to carry on the business in which it is engaged and to own and
     operate the properties used by it in such business as described in the
     Prospectus. The Company's subsidiary constitutes less than 5% of its
     consolidated assets and during the year ended December 31, 1997 contributed
     less than 5% of its consolidated annual operating revenues and net income,
     and the Company does not consider its subsidiary to be material.
 
          (g) The execution and delivery of the Underwriting Agreement have been
     duly authorized by the Company and the Underwriting Agreement constitutes a
     valid and legally binding obligation of the Company; the Bonds have been
     duly authorized, and when issued and delivered pursuant to the Underwriting
     Agreement and the Indenture, will have been duly executed, authenticated,
     issued and delivered and will constitute valid and legally binding
     obligations of the Company in accordance with their respective terms,
     entitled to the benefits provided by the Indenture; the Supplemental
     Indenture has been duly authorized in substantially the form filed as an
     exhibit to the Registration Statement and, when executed and delivered by
     the Company and the Trustee, will constitute a valid and legally binding
     instrument enforceable in accordance with its terms, except to the extent
     the enforceability of the Bonds and the Indenture may be limited by
     bankruptcy, insolvency, reorganization or other laws of general application
     relating to or affecting the enforcement of creditors' rights or general
     equity principles; and the Indenture and the Bonds as executed and
     delivered will conform in all material respects to the descriptions thereof
     in the Prospectus.
 
          (h) The issue and sale of the Bonds and the compliance by the Company
     with all of the provisions of the Bonds, the Indenture, and the
     Underwriting Agreement and the transactions contemplated thereby will not
     conflict with or result in any breach or violation of any of the provisions
     of, or constitute (disregarding any grace or notice period) a default
     under, or result in the imposition of any lien, charge or encumbrance upon
     any property or assets of the Company pursuant to the terms of, any other
     indenture, or any mortgage, loan agreement, contract, note, lease or other
     agreement or instrument to which the Company is a party or by which the
     Company may be bound or to which any of the property or assets of the
     Company is subject, nor will such action result in any violation of the
     provisions of the charter or by-laws of the Company or any statute or any
     order, rule or regulation applicable to the Company of any court or any
     federal, state or other regulatory authority or other governmental body
     having jurisdiction over the Company or any of its properties.
 
          (i) Since the respective dates as of which information is given in the
     Registration Statement and Prospectus and except as may otherwise be stated
     or contemplated therein; (i) there has not been any material adverse change
     in the condition, financial or otherwise, of the Company and its
     subsidiaries considered as one enterprise, or in the earnings, affairs,
     business prospects or properties of the Company and its subsidiaries
     considered as one enterprise, whether or not arising in the ordinary course
     of business or arising from any court or governmental action, order or
     decree, and (ii) there has been no transaction entered into by the Company
     or any subsidiary which is material to the Company and its subsidiaries
     considered as one enterprise, other than transactions in the ordinary
     course of business.
 
          (j) Except as set forth in the Prospectus, the Company, with minor
     exceptions, and subject to noncompliance with certain procedural and other
     requirements in the procurement and granting of gas franchises in a number
     of smaller municipalities formerly served by Mid-Illinois Gas Company, has
     statutory authority, franchises, licenses, rights-of-way, easements and
     consents, free from unduly burdensome restrictions and adequate for the
     conduct of the business in which it is engaged.
 
                                        3
<PAGE>   8
 
          (k) The Illinois Commerce Commission has entered an order authorizing
     the issue and sale of the Bonds by the Company upon terms consistent with
     the Underwriting Agreement, and no other consent, approval, authorization
     or other order or filing with any regulatory or governmental body is
     required for the issuance and sale of the Bonds and consummation of the
     transactions contemplated hereby, except such consents, approvals,
     authorizations, registrations or qualifications as may be required under
     state securities or Blue Sky laws in connection with the purchase and
     distribution of the Bonds by the Underwriters.
 
          (l) The Company is not in violation of its charter or, except as
     disclosed in the Prospectus, in default in the performance or observance of
     any obligation, agreement, covenant or condition contained in any contract,
     indenture, mortgage, loan agreement, note, lease or other instrument to
     which it is a party or by which it or its property is bound or affected
     which is material to the Company and its subsidiary considered as one
     enterprise.
 
          (m) Except as set forth in the Registration Statement and Prospectus,
     there are no legal or governmental proceedings pending to which the Company
     or its subsidiary is a party or of which any property of the Company or its
     subsidiary is the subject, and, to the best of the Company's knowledge, no
     such proceedings are threatened or contemplated by governmental authorities
     or threatened by others, other than proceedings which, if determined
     adversely to the Company and its subsidiary, would not individually or in
     the aggregate have a material adverse effect on the business, properties,
     financial position, net worth or results of operations of the Company and
     its subsidiary considered as a whole.
 
     Any certificate signed by any officer of the Company and delivered to you
or to Underwriters' counsel shall be deemed a representation and warranty by the
Company to each Underwriter as to the statements made therein.
 
     SECTION 3. Purchase, Sale and Delivery of Bonds. Following the execution of
the Underwriting Agreement, the several Underwriters propose to make a public
offering of their respective portions of the Bonds as soon as in the
Representatives' judgment it is advisable upon the terms and conditions set
forth in the Prospectus Supplement.
 
     The Bonds to be purchased by each Underwriter pursuant to the Underwriting
Agreement, in definitive form and registered in such names as the
Representatives may request upon at least forty-eight hours' prior notice to the
Company, shall be delivered by or on behalf of the Company to the
Representatives for the respective accounts of the several Underwriters, against
payment therefor as specified in the Underwriting Agreement in immediately
available funds, at the office of Mayer, Brown & Platt, 190 South LaSalle
Street, Chicago, Illinois 60603 (except as hereinafter provided with respect to
delivery of such Bonds), at the time and date specified in the Underwriting
Agreement or at such other place and time and date as the Representatives and
the Company may agree upon in writing, such time and date being herein called
the "Time of Delivery". If specified by the Representatives in the Underwriting
Agreement, delivery of the Bonds will be made at the Time of Delivery at such
place in New York, New York as shall have been so specified against payment
therefor in Chicago as aforesaid.
 
     SECTION 4. Covenants of the Company. The Company covenants with each
Underwriter that:
 
          (a) The Company will notify the Representatives immediately and
     confirm the notice in writing (i) of the receipt of any request by the
     Commission for any amendment or supplement to the Registration Statement or
     the Prospectus or any amendment or supplement thereto or for additional
     information, and (ii) of the issuance by the Commission of any stop order
     suspending the effectiveness of the Registration Statement or of the
     initiation or threatened initiation of any proceedings for that purpose or
     of the suspension or threatened suspension of the qualification of the
     Bonds for offering or sale in any jurisdiction. The Company will make every
     reasonable effort to prevent the issuance by the Commission of any stop
     order and, if any such stop order shall at any time be issued, to obtain
     the lifting thereof at the earliest moment.
 
          (b) The Company will not file any amendment to the Registration
     Statement or any amendment or supplement to the Prospectus (including a
     prospectus filed pursuant to Rule 424 and including
 
                                        4
<PAGE>   9
 
     documents deemed to be incorporated by reference into the Prospectus)
     without first having furnished the Representatives with a copy of the
     proposed form thereof and given the Representatives a reasonable
     opportunity to review and comment respecting the same and having given
     reasonable consideration to any comments or objections made by the
     Representatives.
 
          (c) The Company will deliver to each of the Representatives, as soon
     as available, one signed copy of the Registration Statement as originally
     filed and of each amendment thereto, including, in each case, documents
     incorporated by reference into the Registration Statement and one set of
     exhibits thereto (other than exhibits incorporated by reference which will
     be furnished upon specific request), and will also deliver to the
     Representatives a reasonable number of conformed copies of the Registration
     Statement as originally filed and of each amendment and post-effective
     amendment thereto including such incorporated documents (without exhibits)
     for each of the Underwriters.
 
          (d) The Company will deliver to each Underwriter from time to time
     during the period when a prospectus is required to be delivered under the
     Act such number of copies of the Prospectus (as amended or supplemented and
     including incorporated documents) as the Representatives may reasonably
     request for the purposes contemplated by the Act or the Regulations;
     provided, however, that the delivery of copies of the Prospectus (as
     amended or supplemented and including incorporated documents) more than
     nine months after the date of the Underwriting Agreement shall be at the
     expense of the Underwriter requesting such delivery.
 
          (e) During the period when a prospectus is required to be delivered
     under the Act, the Company will comply so far as it is able, and at its own
     expense (for a period not to exceed nine months), with all requirements
     imposed upon it by the Act, and by Sections 13 and 14 of the 1934 Act, as
     now or hereafter amended, and by the Regulations, as from time to time in
     force, so far as necessary to permit the continuance of sales of or dealing
     in the Bonds during such period in accordance with the provisions hereof
     and of the Prospectus.
 
          (f) If any event shall occur as a result of which it is necessary, in
     the opinion of counsel for the Company and of Underwriters' counsel, to
     amend or supplement the Prospectus in order to make the Prospectus not
     misleading in the light of the circumstances existing at the time it is
     delivered to a purchaser, or if it is necessary to amend or supplement the
     Prospectus to comply with law, the Company will forthwith prepare and
     furnish to the Underwriters, without expense to them except as otherwise
     provided in subsection (d) of this Section 4, a reasonable number of copies
     of an amendment or amendments or a supplement or supplements to the
     Prospectus (in the form referred to in subsection (b) of this Section 4)
     which will amend or supplement the Prospectus so that as amended or
     supplemented it will not contain any untrue statement of a material fact or
     omit to state any material fact necessary in order to make the statements
     therein not misleading, or so that the Prospectus will comply with law. For
     the purposes of this subsection, the Company will furnish such information
     as the Representatives may from time to time reasonably request.
 
          (g) The Company will endeavor in good faith, in cooperation with the
     Underwriters, to qualify the Bonds for offering and sale under the
     applicable securities laws of such jurisdictions as the Representatives may
     designate; provided, however, that the Company shall not be obligated to
     file any general consent to service or to qualify as a foreign corporation
     or as a dealer in securities in any jurisdiction in which it is not so
     qualified. In each jurisdiction where any of the Bonds shall be qualified
     as above provided, the Company will make and file such statements and
     reports in each year as are or may be reasonably required by the laws
     thereof.
 
          (h) The Company will make generally available to its security holders
     as soon as practicable, but not later than 75 days after the close of the
     period covered thereby, an earnings statement (in form complying with the
     provisions of Section 11(a) of the Act and the Regulations thereunder
     (including, at the option of the Company, Rule 158), which need not be
     certified by independent public accountants unless required by the Act or
     the Regulations), covering a twelve-month period beginning on the first day
     of the calendar quarter following the Time of Delivery.
 
                                        5
<PAGE>   10
 
     (i) The Company agrees that it will not publicly offer or sell any
intermediate or long-term debt between the date of the Underwriting Agreement
and Time of Delivery without the prior written consent of the Representatives.
 
     SECTION 5. Payment of Expenses. The Company will pay all expenses incident
to the performance of its obligations under the Underwriting Agreement,
including (i) the printing and filing by the Company of the registration
statement and the printing of the Underwriting Agreement, any Agreement Among
Underwriters, any Selling Agreement, the Supplemental Indenture and the
Underwriters' Questionnaire, (ii) the authorization, issuance and delivery of
the Bonds to the Underwriters, including the printing and engraving of the
Bonds, and all taxes, if any, upon the issuance and sale of the Bonds to the
Underwriters, (iii) the qualification of the Bonds under the securities laws of
the various jurisdictions in accordance with the provisions of subsection (g) of
Section 4, including filing fees and fees and disbursements of Underwriters'
counsel in connection with such qualification and in connection with the
preparation of the Blue Sky Survey (such fees of Underwriters' counsel not to
exceed $      in the aggregate), (iv) any fees charged by securities rating
services for rating the Bonds, (v) the fees and expenses of the Trustee and its
counsel in connection with the Bonds and the Supplemental Indenture, (vi) the
printing and delivery to the Underwriters and dealers in quantities as
hereinbefore stated of copies of the registration statement and all amendments
thereto, of any preliminary prospectuses and amended preliminary prospectuses,
of the Registration Statement and any amendments thereto, and of the Prospectus
and any amendments or supplements thereto, and (vii) the cost of printing and
delivery to the Underwriters of copies of the Blue Sky Survey.
 
     If this Agreement is terminated by the Representatives in accordance with
the provisions of Section 6 or Section 10(b), or is prevented by the Company
from becoming effective in accordance with the provisions of Section 10(a), the
Company shall reimburse the Underwriters severally for their out-of-pocket
expenses, including the reasonable fees and disbursements of counsel for the
Underwriters incurred in connection with the offering.
 
     SECTION 6. Conditions of Underwriters' Obligations. The several obligations
of the Underwriters hereunder are subject to the accuracy of and compliance with
the representations and warranties of the Company herein contained, to the
performance by the Company of its obligations hereunder and to the following
further conditions:
 
          (a) At the Time of Delivery no stop order suspending the effectiveness
     of the Registration Statement shall have been issued under the Act or
     proceedings therefor initiated or threatened by the Commission.
 
          (b) At the Time of Delivery the Representatives shall have received:
 
             (1) The favorable opinion, dated as of the Time of Delivery, of
        Mayer, Brown & Platt, counsel for the Company, in form and substance
        satisfactory to counsel for the Underwriters, to the effect that:
 
                (i) the Company is a corporation in good standing, duly
           organized and validly existing under the laws of the State of
           Illinois and has due corporate authority to carry on the business in
           which it is engaged and to own and operate the properties used by it
           in such business;
 
                (ii) the Indenture is in due and proper form, has been duly and
           validly authorized by the necessary corporate action and by orders
           duly entered by the Illinois Commerce Commission; no authorization,
           approval, consent, certificate or order of any other state commission
           or regulatory authority or of any federal commission or regulatory
           authority not already obtained is required in respect of the
           execution and delivery of the Indenture; and the Indenture has been
           duly and validly executed and delivered and is a valid and
           enforceable instrument in accordance with its terms, except as
           enforcement of provisions of the Indenture may be limited by
           bankruptcy or other laws of general application affecting the
           enforcement of creditors' rights and by general equity principles;
 
                                        6
<PAGE>   11
 
                (iii) the Bonds are in due and proper form; the issue and sale
           of the Bonds by the Company in accordance with the terms of the
           Underwriting Agreement have been duly and validly authorized by the
           necessary corporate action and by order duly entered by the Illinois
           Commerce Commission; no authorization, approval, consent, certificate
           or order of any other state commission or regulatory authority or of
           any federal commission or regulatory authority not already obtained
           is required in respect of such issue and sale (except such consents,
           approvals, authorizations, registrations or qualifications as may be
           required under state securities or Blue Sky laws in connection with
           the purchase and distribution of the Bonds by the Underwriters); the
           Bonds have been duly executed and delivered to the Underwriters
           against payment of the agreed consideration therefor and, assuming
           due authentication thereof by the Trustee, constitute valid and
           enforceable obligations of the Company in accordance with their
           terms, secured by the lien of and, with like exception as noted in
           the foregoing subdivision (ii), entitled to the benefits provided by
           the Indenture, and the registered owners of the Bonds will be
           entitled to the payment of principal and interest, and premium in
           case of redemption, as therein provided; the Bonds and the Indenture
           conform as to legal matters in all material respects with the
           statements concerning them made in the Prospectus, and such
           statements accurately set forth the matters respecting the Bonds and
           the Indenture required to be set forth in the Prospectus;
 
                (iv) The Registration Statement is effective under the Act and
           the Indenture has been duly qualified under the Trust Indenture Act,
           and to the best of the knowledge of said counsel no proceedings for a
           stop order are pending or threatened under Section 8(d) of the Act;
 
                (v) the execution and delivery of the Underwriting Agreement by
           the Company has been duly authorized by the necessary corporate
           action, and the Underwriting Agreement has been duly executed and
           delivered by the Company;
 
                (vi) the Company has good and sufficient title to all property
           described or referred to in the Indenture and purported to be
           conveyed thereby (except property released from the lien of the
           Indenture in connection with the sale or other disposition thereof),
           subject only to the lien of the Indenture and to permitted liens as
           defined therein; the Indenture has been duly filed for recordation in
           such manner and in such places as is required by law in order to give
           constructive notice of, establish, preserve and protect the lien of
           the Indenture; the Indenture constitutes a valid, direct first
           mortgage lien, subject only to permitted liens, on substantially all
           property of the Company, except property expressly excepted by the
           terms of the Indenture; the Indenture will, when recorded or
           registered by the Company in accordance with its covenants under the
           Indenture, constitute a valid, direct first mortgage lien on all
           property of the character of that now subject to the lien of the
           Indenture hereafter acquired by the Company, subject only to
           permitted liens and to liens, if any, existing or placed on such
           after-acquired property at the time of the acquisition thereof;
 
                (vii) the issue and sale of the Bonds and the compliance by the
           Company with all of the provisions of the Bonds, the Indenture and
           the Underwriting Agreement will not conflict with or result in a
           breach or violation of any of the provisions of, or constitute
           (disregarding any grace or notice period) a default under, any
           indenture, mortgage, loan agreement, contract, note, lease or other
           agreement or instrument, known to such counsel, to which the Company
           is a party or by which the Company is bound or to which any of the
           property or assets of the Company is subject (with such exceptions as
           are in the aggregate not material to the business or financial
           condition of the Company or the validity of the Bonds), nor will such
           action result in any violation of the provisions of the Charter or
           By-Laws of the Company, or, to the best of their knowledge, any
           statute or any order, rule or regulation applicable to the Company of
           any court or governmental agency or body having jurisdiction over the
           Company or any of its properties (except such consents, approvals,
           authorizations, registrations or qualifications as may be required
           under state securities or Blue Sky laws in connection with the
           purchase and distribution of the Bonds by the Underwriters);
 
                                        7
<PAGE>   12
 
                (viii) at the time the registration statement became effective,
           the registration statement and the related prospectus (other than the
           financial statements and notices thereto and supporting schedules and
           other financial information included therein, as to which no opinion
           need be rendered) complied as to form in all material respects with
           the requirements of the Act and the Trust Indenture Act and the
           Regulations;
 
                (ix) with minor exceptions, and subject to noncompliance with
           certain procedural and other requirements in the procurement and
           granting of gas franchises in a number of smaller municipalities
           formerly served by Mid-Illinois Gas Company, the Company holds
           franchises from all of the incorporated cities and villages included
           in the communities in which the Company renders gas service; all of
           the franchises so held by the Company are valid and subsisting and
           authorize it to engage in the business conducted by it in the
           respective municipalities granting such franchises; the Company also
           holds certificates of public convenience and necessity issued by the
           Illinois Commerce Commission, which are valid and subsisting and
           constitute due authorization by such commission for the conduct by
           the Company of its operations in all areas served;
 
                (x) to the best of their knowledge and information, there are no
           contracts, indentures, mortgages, loan agreements, notes, leases or
           other instruments of a character required to be described in the
           Registration Statement or Prospectus or to be filed as exhibits to
           the Registration Statement other than those described therein or
           filed or incorporated by reference as exhibits thereto and the
           descriptions thereof or reference thereto are correct; and
 
                (xi) except as disclosed in the Prospectus, there are no
           material pending or threatened legal proceedings, considering the
           Company and the subsidiaries as a single enterprise, known to said
           counsel, to which the Company or any subsidiary is a party or of
           which property of the Company or any subsidiary is the subject, and
           to the best of the knowledge of said counsel there are no such
           proceedings contemplated by governmental authorities.
 
        Such counsel shall further state that, based upon their participation in
        the preparation of the Registration Statement and the Prospectus, and
        any amendment or supplement thereto, and upon their review and
        discussions of the contents thereof, but without independent check or
        verification except as specified, nothing has come to their attention
        that has caused them to believe that the Registration Statement, at the
        time it became effective, contained an untrue statement of a material
        fact or omitted to state a material fact required to be stated therein
        or necessary to make the statements therein not misleading or that the
        Prospectus, and any amendment or supplement thereto, at the date the
        Registration Statement became effective, the date of this Agreement or
        at the Time of Delivery, contained an untrue statement of a material
        fact or omitted to state a material fact necessary in order to make the
        statements therein, in light of the circumstances under which they were
        made, not misleading.
 
             (2) The favorable opinion of Wildman, Harrold, Allen & Dixon,
        counsel for the Underwriters, with respect to the incorporation of the
        Company, the validity of the Bonds and the Indenture, the Registration
        Statement, the Prospectus and other related matters as the
        Representatives may reasonably request; provided that any opinion
        requested with respect to the jurisdiction of regulatory authorities
        (other than the Illinois Commerce Commission, the Securities and
        Exchange Commission and state securities or Blue Sky authorities) and
        the matters in subdivisions (vi) and (ix) above will rely upon the
        opinion of Mayer, Brown & Platt.
 
          (c) At the effective date of the Registration Statement and at the
     Time of Delivery the Representatives shall have received a letter from
     Arthur Andersen LLP, dated the effective date or Time of Delivery,
     respectively, in form and substance satisfactory to the Representatives,
     advising that (i) they are independent public accountants with respect to
     the Company and its subsidiaries as required by the Act and the 1934 Act
     and the applicable Regulations, (ii) in their opinion, the audited
     consolidated financial statements and any supplemental financial
     information and schedules of the Company examined by them and incorporated
     by reference in the Registration Statement and Prospectus comply as to form
 
                                        8
<PAGE>   13
 
     in all material respects with the applicable accounting requirements of the
     Act, the 1934 Act and the applicable Regulations, (iii) on the basis of a
     reading of the latest available unaudited interim consolidated financial
     statements prepared by the Company, a reading of the minutes of meetings of
     the shareholder and the board of directors and executive committee of the
     Company and its subsidiaries, consultation with officers of the Company
     responsible for financial and accounting matters and other specified
     procedures, nothing has come to their attention which caused them to
     believe that (A) the unaudited interim condensed consolidated financial
     statements included or incorporated by reference in the Prospectus do not
     comply as to form in all material respects with the applicable accounting
     requirements of the Act, the 1934 Act and the applicable Regulations or are
     not in conformity with generally accepted accounting principles applied on
     a basis substantially consistent with that of the audited financial
     statements incorporated as aforesaid, (B) the unaudited income statement
     data and balance sheet data (other than such data for the periods referred
     to in (A) above) included or incorporated by reference in the Prospectus do
     not agree with the corresponding items in the audited or unaudited, as the
     case may be, financial statements from which such data were derived or were
     not determined on a basis substantially consistent with that of the
     corresponding amounts included in the audited consolidated financial
     statements of the Company incorporated in the Registration Statement and
     Prospectus, or (C) at a specified date within five business days of the
     date of such letter with respect to (1) below, and during the period from
     the date of the latest audited consolidated financial statements or
     unaudited interim condensed consolidated financial statements, as the case
     may be, incorporated in the Prospectus to the date of the latest available
     unaudited interim consolidated financial statements (if any) prepared by
     the Company with respect to (2) below, except in all instances as set forth
     in or contemplated by the Prospectus or as set forth in such letter: (1)
     there was any increase in the consolidated long-term debt of the Company
     and its subsidiaries, as compared with the amounts set forth in the latest
     balance sheet included or incorporated by reference in the Prospectus, or
     (2) there were any decreases in consolidated operating income or net income
     as compared with the corresponding period in the preceding year; and (iv)
     they have carried out specified procedures performed for the purpose of
     comparing certain financial information and percentages (which is limited
     to financial information derived from general accounting records of the
     Company) specified by the Representatives and appearing in the Registration
     Statement or in schedules or exhibits to the Registration Statement or in
     the Prospectus or in documents incorporated by reference in the Prospectus
     with indicated amounts in the financial statements or accounting records of
     the Company and (excluding any questions of legal interpretation and, in
     the case of the letter delivered at the Time of Delivery, any exceptions
     disclosed in the letter delivered at the Effective Date) have found such
     information and percentages to be in agreement with the relevant accounting
     and financial information of the Company referred to in such letter in the
     description of the procedures performed by them. If such letter discloses
     any material adverse decreases or increases, as the case may be, in the
     items specified in item (iii) (C) above which are not set forth in or
     contemplated by the Prospectus which, in the judgment of the
     Representatives, makes it impracticable or inadvisable to proceed with the
     public offering or the delivery of the Bonds on the terms and in the manner
     contemplated by the Prospectus, this Agreement and all obligations of the
     Underwriters hereunder may be cancelled by the Representatives by notifying
     the Company in the manner and with the effect provided below in the last
     sentence of this Section 6.
 
          (d) At the Time of Delivery the Representatives shall have received a
     certificate of the Chairman, President, Vice President and principal
     financial officer, Vice President and principal accounting officer or
     Treasurer of the Company, dated as of the Time of Delivery, to the effect
     that the signer of such certificate has carefully examined the Registration
     Statement, the Prospectus and any amendment or supplement thereto and the
     Underwriting Agreement and that, in his opinion, at the time the
     Registration Statement became effective, the Registration Statement did not
     contain an 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 therein not misleading, and at the date of the Underwriting
     Agreement the Prospectus did not contain an 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 therein not misleading, and since
     the date of the
 
                                        9
<PAGE>   14
 
     Underwriting Agreement, no event has occurred which should have been set
     forth in an amendment of or supplement to the Prospectus which has not been
     so set forth; and no stop order suspending the effectiveness of the
     Registration Statement has been issued and no proceedings therefor have
     been instituted or threatened by the Commission; and to the further effect
     that all the representations and warranties contained in Section 2 hereof
     are true and correct, with the same force and effect as though expressly
     made at the Time of Delivery.
 
          (e) At the Time of Delivery counsel for the Underwriters shall have
     been furnished with such documents and opinions as they may reasonably
     require for the purpose of enabling them to pass upon the sale of the Bonds
     as herein contemplated and related proceedings, or in order to evidence the
     accuracy or completeness of any of the representations or warranties, or
     the fulfillment of any of the conditions, herein contained; and all
     proceedings taken by the Company in connection with the sale of the Bonds
     as herein contemplated shall be satisfactory in form and substance to the
     Representatives and counsel for the Underwriters.
 
     If any of the conditions specified in this Section shall not have been
fulfilled when and as required by this Agreement to be fulfilled, this Agreement
and all obligations of the Underwriters hereunder may be cancelled by the
Representatives by notifying the Company of such cancellation in writing or by
telecopy at any time at or prior to the Time of Delivery and any such
cancellation shall be without liability of any party to any other party except
as otherwise provided in this Agreement.
 
     SECTION 7. Condition of Company's Obligations. The obligations of the
Company to sell and deliver the Bonds are subject to the following conditions:
that at the Time of Delivery no stop order suspending the effectiveness of the
Registration Statement shall have been issued or proceedings therefor initiated
or threatened; that the order of the Illinois Commerce Commission, referred to
in Section 2(k), shall be in full force and effect substantially in the form in
which such order shall originally have been entered; and that the Indenture
shall be qualified under the Trust Indenture Act.
 
     SECTION 8. Indemnification. (a) The Company agrees to indemnify and hold
harmless each Underwriter and each person, if any, who controls any Underwriter
within the meaning of the Act or the 1934 Act, as follows:
 
          (i) against any and all loss, liability, claim, damage and expense,
     whatsoever, arising out of any untrue statement or alleged untrue statement
     of a material fact contained in the registration statement as it became
     effective, or in any amendment thereto, or in the Registration Statement
     (or any amendment thereto), or the omission or alleged omission therefrom
     of a material fact required to be stated therein or necessary to make the
     statements therein not misleading, or arising out of any untrue statement
     or alleged untrue statement of a material fact contained in any preliminary
     prospectus or the Prospectus (or any amendment or supplement thereto) or
     the omission or alleged omission therefrom of a material fact necessary in
     order to make the statements therein, in light of the circumstances under
     which they were made, not misleading, unless such untrue statement or
     omission or such alleged untrue statement or omission was made in reliance
     upon and in conformity with written information respecting the Underwriters
     furnished to the Company by or on behalf of any Underwriter through the
     Representatives expressly for use in the Registration Statement (or any
     amendment thereto) or the Prospectus (or any amendment or supplement
     thereto);
 
          (ii) against any and all loss, liability, claim, damage and expense
     whatsoever to the extent of the aggregate amount paid in settlement of any
     litigation, commenced or threatened, or of any claim whatsoever based upon
     any such untrue statement or omission or any such alleged untrue statement
     or omission, if such settlement is effected with the written consent of the
     Company; and
 
          (iii) against any and all expenses whatsoever reasonably incurred in
     investigating, preparing or defending against any litigation, commenced or
     threatened, or any claim whatsoever based upon any such untrue statement or
     omission, or any such alleged untrue statement or omission, to the extent
     that any such expense is not paid under (i) or (ii) above, and, in the case
     of (i) above, unless such untrue statement or omission or such alleged
     untrue statement or omission was made in reliance upon and in
 
                                       10
<PAGE>   15
 
     conformity with written information respecting the Underwriters furnished
     to the Company by or on behalf of any Underwriter through the
     Representatives expressly for use in the Registration Statement (or any
     amendment thereto) or the Prospectus (or any amendment or supplement
     thereto), or, in the case of (ii) above, provided such settlement is
     effected with the written consent of the Company.
 
     This indemnity agreement is subject to the condition that, insofar as it
relates to any untrue statement, alleged untrue statement, omission or alleged
omission made in a preliminary prospectus or preliminary prospectus supplement,
but eliminated or remedied in the Prospectus, such indemnity agreement shall not
inure to the benefit of any Underwriter from whom the person asserting any loss,
liability, claim or damage purchases the Bonds which are the subject thereof (or
to the benefit of any person who controls such Underwriter) if such Underwriter
fails to send or give a copy of the Prospectus (excluding documents incorporated
by reference) to such person prior to or together with written confirmation of
the sale of such Bonds to such person and the delivery thereof would have
constituted a defense to the claim by such person.
 
     In no case shall the Company be liable under this indemnity agreement with
respect to any claim made against any Underwriter or any such controlling person
unless the Company shall be notified in writing of the nature of the claim
within a reasonable time after the assertion thereof, but failure to so notify
the Company shall not relieve it from any liability which it may have otherwise
than on account of this indemnity agreement. The Company shall be entitled to
participate at its own expense in the defense, or, if it so elects, within a
reasonable time after receipt of such notice, to assume the defense of any suit
brought to enforce any such claim, but if it so elects to assume the defense,
such defense shall be conducted by counsel chosen by it and approved by the
Underwriter or Underwriters or controlling person or persons, defendant or
defendants in any suit so brought, which approval shall not be unreasonably
withheld. In the event that the Company elects to assume the defense of any such
suit and retains such counsel, the Underwriter or Underwriters or controlling
person or persons, defendant or defendants in the suit shall thereafter bear the
fees and expenses of any additional counsel retained by them. In the event that
the parties to any such action (including impleaded parties) include both the
Company and one or more Underwriters and any such Underwriter shall have been
advised by counsel chosen by it and satisfactory to the Company that there may
be one or more legal defenses available to it which are different from or
additional to those available to the Company, the Company shall not have the
right to assume the defense of such action on behalf of such Underwriter and
will reimburse such Underwriter and any person controlling such Underwriter as
aforesaid for the reasonable fees and expenses of any counsel retained by them,
it being understood that the Company shall not, in connection with any one
action or separate but similar or related actions in the same jurisdiction
arising out of the same general allegations or circumstances, be liable for the
reasonable fees and expense of more than one separate firm of attorneys for all
such Underwriters and controlling persons, which firm shall be designated in
writing by the Representatives. The Company agrees to notify the Representatives
within a reasonable time of the assertion of any claim against it, any of its
officers or directors or any person who controls the Company within the meaning
of the Act or the 1934 Act, in connection with the sale of the Bonds.
 
     (b) Each Underwriter severally agrees that it will indemnify and hold
harmless the Company, its directors, and each of its officers who signed the
Registration Statement and each person, if any, who controls the Company within
the meaning of the Act or the 1934 Act, to the same extent as the indemnity
contained in subsection (a) of this Section, but only with respect to statements
or omissions made in the registration statement as it became effective, or in
any amendment thereto, or in the Registration Statement (or any amendment
thereto) or the Prospectus (or any amendment or supplement thereto) in reliance
upon and in conformity with written information respecting the Underwriters
furnished to the Company by or on behalf of such Underwriter through the
Representatives expressly for use in the Registration Statement (or any
amendment thereto) or the Prospectus (or any amendment or supplement thereto).
In case any action shall be brought against the Company or any person so
indemnified based on the Registration Statement (or any amendment thereto) or
the Prospectus (or any amendment or supplement thereto) and in respect of which
indemnity may be sought against any Underwriter, such Underwriter shall have the
rights and duties given to the Company, and the Company and each person so
indemnified shall have the rights and duties given to the Underwriters, by the
provisions of subsection (a) of this Section.
 
                                       11
<PAGE>   16
 
     (c) The obligations of the Company under this Section 8 shall be in
addition to any liability which the Company may otherwise have and shall extend,
upon the same terms and conditions, to each person, if any, who controls any
Underwriter within the meaning of the Act or the 1934 Act; and the obligations
of the Underwriters under this Section 8 shall be in addition to any liability
which the respective Underwriters may otherwise have and shall extend, upon the
same terms and conditions, to each officer and director of the Company and to
each person, if any, who controls the Company within the meaning of the Act or
the 1934 Act.
 
     SECTION 9. Representations, Warranties and Agreements to Survive
Delivery. All representations, warranties and agreements contained in the
Underwriting Agreement and/or contained in certificates of officers of the
Company submitted pursuant hereto, shall remain operative and in full force and
effect, regardless of any investigation made by or on behalf of any Underwriter
or any controlling person of any Underwriter, or by or on behalf of the Company,
and shall survive payment for and delivery of the Bonds.
 
     SECTION 10. Effective Date of the Underwriting Agreement and Termination
Thereof. (a) The Underwriting Agreement shall become effective at the time of
the initial public offering by the Underwriters of any of the Bonds. The time of
the initial public offering shall mean 12:00 noon, New York City time, on the
first full business day after the Underwriting Agreement is executed or at such
time as the Representatives may authorize the sale of the Bonds to the public by
the Underwriters or other securities dealers, whichever shall first occur. The
Representatives or the Company may prevent the Underwriting Agreement from
becoming effective without liability of any party to any other party, except as
otherwise provided in the Underwriting Agreement, by giving the notice indicated
below in this Section prior to the time the Underwriting Agreement would
otherwise become effective as herein provided.
 
     (b) The Representatives shall have the right to terminate the Underwriting
Agreement by giving the notice indicated below in this Section at any time at or
prior to the Time of Delivery if (i) the Company shall have sustained since the
respective dates as of which information is given in the Prospectus any material
loss or interference with its business from fire, explosion, flood or other
calamity, whether or not covered by insurance, or from any labor dispute or
court or governmental action, order or decree; or (ii) since the respective
dates as of which information is given in the Prospectus there shall have been
any material increase in the long-term debt, or any material adverse change, or
any development involving a prospective material adverse change, in or affecting
the general business affairs, management, financial position, results of
operations, or business prospects of the Company and its subsidiaries considered
as one enterprise, otherwise than as set forth or contemplated in the
Prospectus, the effect of which, in any such case described in clause (i) or
(ii), in the judgment of the Representatives makes it impracticable or
inadvisable to proceed with the public offering or the delivery of the Bonds on
the terms and in the manner contemplated in the Prospectus; or (iii) there shall
have occurred the outbreak or escalation of hostilities involving in a
significant way the armed forces of the United States, or the declaration by the
United States, on or after the date of the Underwriting Agreement, of a national
emergency or war, or there shall have occurred a general suspension or
limitation of trading in securities on the New York or American Stock Exchanges,
or the establishment of minimum prices on either such Exchange, or a general
moratorium on commercial banking activities in New York is declared by either
federal or New York state authorities, the effect of which in the judgment of
the Representatives makes it impracticable or inadvisable to proceed with the
public offering or the delivery of the Bonds on the terms and in the manner
contemplated in the Prospectus. If the Representatives shall so terminate the
Underwriting Agreement, such termination shall be without liability of any party
to any other party except as otherwise provided in the Underwriting Agreement.
 
     (c) If the Representatives elect to prevent the Underwriting Agreement from
becoming effective or to terminate the Underwriting Agreement as provided in
this Section, the Company and each other Underwriter shall be notified promptly
by the Representatives, by telephone or telegram, confirmed by letter. If the
Company elects to prevent the Underwriting Agreement from becoming effective as
provided in this Section, the Representatives shall be notified promptly by the
Company by telephone or telegram, confirmed by letter.
 
     SECTION 11. Default of Underwriters. If any one or more of the Underwriters
shall fail at the Time of Delivery to purchase the amount of Bonds which it or
they are obligated to purchase hereunder (the
 
                                       12
<PAGE>   17
 
"Defaulted Bonds"), then the Representatives shall have the right, within 24
hours thereafter, to make arrangements for one or more of the non-defaulting
Underwriters, or any other underwriters, to purchase all, but not less than all,
of the Defaulted Bonds in such amounts as may be agreed upon and upon the terms
herein set forth. If, however, during such 24 hours the Representatives shall
not have completed such arrangements for the purchase of all of the Defaulted
Bonds, then the Company shall be entitled to a further period of 24 hours within
which to procure another party of parties satisfactory to the Representatives to
purchase all of such Defaulted Bonds on such terms. If, after giving effect to
any arrangements for the purchase of Defaulted Bonds by the Representatives and
the Company as provided above, then:
 
          (a) if the amount of Defaulted Bonds does not exceed 10% of the
     aggregate principal amount of the Bonds being sold hereunder, the
     non-defaulting Underwriters shall be obligated to purchase severally the
     full amount thereof in the proportions that their respective underwriting
     obligations hereunder bear to the underwriting obligations of all
     non-defaulting Underwriters, or
 
          (b) if the amount of Defaulted Bonds exceeds 10% of the aggregate
     principal amount of the Bonds being sold hereunder, the Underwriting
     Agreement shall terminate without any liability on the part of the Company
     or any non-defaulting Underwriter.
 
     The termination of the Underwriting Agreement pursuant to this Section
shall be without liability on the part of the Company or any of said
non-defaulting Underwriters, except for the respective obligations of the
Company and the Underwriters pursuant to Section 8 and except that the Company
shall be obligated to reimburse the Underwriters for their out-of-pocket
expenses (including reasonable fees and disbursements of counsel for the
Underwriters) incurred in connection with the offering if the Underwriting
Agreement could have been terminated by the Representatives pursuant to Section
6 or 10(b).
 
     Nothing herein shall relieve any Underwriter so defaulting from liability,
if any, for such default.
 
     In the event of a default by any one or more Underwriters as set forth in
this Section, either the Representatives or the Company shall have the right to
postpone the Time of Delivery for an additional period not exceeding 7 days in
order that any required changes in the Registration Statement and Prospectus or
in any other documents or arrangements may be effected.
 
     SECTION 12. Notices. Except as otherwise provided in the Underwriting
Agreement, all communications under the Underwriting Agreement shall be in
writing, and, if sent to the Underwriters, shall be mailed, delivered or
telecopied and confirmed to the address of the Representatives, as set forth in
the Underwriting Agreement (except that any notice to an Underwriter pursuant to
Section 8 hereof shall be sent to it at its address set forth in the copies of
the Underwriters' Questionnaires furnished to the Company), or, if sent to the
Company shall be mailed or telecopied and confirmed to it at P.O. Box 190,
Aurora, Illinois 60507-0190, or delivered to it at 1844 Ferry Road, Naperville,
Illinois, for the attention of Donald W. Lohrentz, Vice President.
 
     SECTION 13. Parties. The Underwriting Agreement shall inure to the benefit
of and be binding upon the Underwriters and the Company and their respective
successors. Nothing expressed or mentioned in the Underwriting Agreement is
intended or shall be construed to give any person, firm or corporation, other
than the parties hereto and their respective successors and the controlling
persons and the directors and officers referred to in Section 8, any legal or
equitable right, remedy or claim under or in respect of the Underwriting
Agreement or any provision herein contained; the Underwriting Agreement and all
conditions and provisions hereof being intended to be and being for the sole and
exclusive benefit of the parties hereto and their respective successors and said
controlling persons, directors and officers and for the benefit of no other
person, firm or corporation. No purchaser of any Bonds from any Underwriter
shall be deemed to be a successor by reason merely of such purchase.
 
     SECTION 14. Choice of Law. The Underwriting Agreement shall be construed in
accordance with, and governed by, the laws of the State of Illinois.
 
                                       13

<PAGE>   1
 
                                                                    EXHIBIT 4.20
================================================================================
 
                             SUPPLEMENTAL INDENTURE
                            ------------------------
                            DATED
                            ------------------------
                         NORTHERN ILLINOIS GAS COMPANY
                                       TO
                         HARRIS TRUST AND SAVINGS BANK
                      TRUSTEE UNDER INDENTURE DATED AS OF
                        JANUARY 1, 1954 AND SUPPLEMENTAL
                               INDENTURES THERETO
 
                            ------------------------
 
                              FIRST MORTGAGE BONDS
                                   % SERIES DUE
 
================================================================================
This instrument was prepared by Donald W. Lohrentz, 1844 Ferry Road, Naperville,
                              Illinois 60563-9600.
<PAGE>   2
 
THIS SUPPLEMENTAL INDENTURE, dated the       day of        ,        , between
     NORTHERN ILLINOIS GAS COMPANY, a corporation organized and existing under
     the laws of the State of Illinois (hereinafter called the "Company"), and
     HARRIS TRUST AND SAVINGS BANK, an Illinois Banking Corporation,
     (hereinafter called the "Trustee"), as Trustee under an Indenture dated as
     of January 1, 1954, as supplemented by Supplemental Indentures dated,
     respectively, February 9, 1954, April 1, 1956, June 1, 1959, July 1, 1960,
     June 1, 1963, July 1, 1963, August 1, 1964, August 1, 1965, May 1, 1966,
     August 1, 1966, July 1, 1967, June 1, 1968, December 1, 1969, August 1,
     1970, June 1, 1971, July 1, 1972, July 1, 1973, April 1, 1975, April 30,
     1976, April 30, 1976, July 1, 1976, August 1, 1976, December 1, 1977,
     January 15, 1979, December 1, 1981, March 1, 1983, October 1, 1984,
     December 1, 1986, March 15, 1988, July 1, 1988, July 1, 1989, July 15,
     1990, August 15, 1991, July 15, 1992, February 1, 1993, March 15, 1993, May
     1, 1993, July 1, 1993, August 15, 1994, October 15, 1995, May 10, 1996,
     August 1, 1996, June 1, 1997 and October 15, 1997, such Indenture dated as
     of January 1, 1954, as so supplemented, being hereinafter called the
     "Indenture."
 
WITNESSETH:
 
     WHEREAS, the Indenture provides for the issuance from time to time
thereunder, in series, of bonds of the Company for the purposes and subject to
the limitations therein specified; and
 
     WHEREAS, the Company desires, by this Supplemental Indenture, to create an
additional series of bonds to be issuable under the Indenture, such bonds to be
designated "First Mortgage Bonds,      % Series due             " (hereinafter
called the "bonds of this Series"), and the terms and provisions to be contained
in the bonds of this Series or to be otherwise applicable thereto to be as set
forth in this Supplemental Indenture; and
 
     WHEREAS, the forms, respectively, of the bonds of this Series, and
Trustee's certificate to be endorsed on all bonds of this Series, are to be
substantially as follows:
 
                             (FORM OF FACE OF BOND)
NO. RU _____                                                           $________
 
                         NORTHERN ILLINOIS GAS COMPANY
 
                    FIRST MORTGAGE BOND,       % SERIES DUE
 
     NORTHERN ILLINOIS GAS COMPANY, an Illinois corporation (hereinafter called
the "Company"), for value received, hereby promises to pay to
                     or registered assigns, the sum of
              Dollars, on the       day of             , and to pay to the
registered owner hereof interest on said sum from the date hereof until said sum
shall be paid, at the rate of              per centum (      %) per annum,
payable semiannually on the        day of             and the        day of
            in each year. Both the principal of and the interest on this bond
shall be payable at the office or agency of the Company in the City of Chicago,
State of Illinois, or, at the option of the registered owner, at the office or
agency of the Company in the Borough of Manhattan, The City and State of New
York, in any coin or currency of the United States of America which at the time
of payment is legal tender for the payment of public and private debts. Any
installment of interest on the bonds may, at the Company's option, be paid by
mailing checks for such interest payable to or upon the written order of the
person entitled thereto to the address of such person as it appears on the
registration books.
 
     So long as there is no existing default in the payment of interest on this
bond, the interest so payable on any interest payment date will be paid to the
person in whose name this bond is registered on the        or the
(whether or not a business day), as the case may be, next preceding such
interest payment date. If and to the extent that the Company shall default in
the payment of interest due on such interest payment date, such defaulted
interest shall be paid to the person in whose name this bond is registered on
the record date fixed, in advance, by the Company for the payment of such
defaulted interest.
 
     Additional provisions of this bond are set forth on the reverse hereof.
<PAGE>   3
 
     This bond shall not be entitled to any security or benefit under the
Indenture or be valid or become obligatory for any purpose unless and until it
shall have been authenticated by the execution by the Trustee, or its successor
in trust under the Indenture, of the certificate endorsed hereon.
 
     IN WITNESS WHEREOF, Northern Illinois Gas Company has caused this bond to
be executed in its name by its Chairman, President, or a Vice President,
manually or by facsimile signature, and has caused its corporate seal to be
impressed hereon or a facsimile thereof to be imprinted hereon and to be
attested by its Secretary or its Assistant Secretary, manually or by facsimile
signature.
 
Dated
                                            NORTHERN ILLINOIS GAS COMPANY
 
                                            By
 
                                            ------------------------------------
                                             President
ATTEST:
 
- ------------------------------------
             Secretary
 
               (FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION)
 
     This bond is one of the bonds of the series designated therein, referred to
and described in the within-mentioned Supplemental Indenture dated
 
HARRIS TRUST AND SAVINGS,
TRUSTEE
 
By
 
    --------------------------------
           Authorized Officer
 
                         (FORM OF REVERSE SIDE OF BOND)
 
     This bond is one, of the series hereinafter specified, of the bonds issued
and to be issued in series from time to time under and in accordance with and
secured by an Indenture dated as of January 1, 1954, to Harris Trust and Savings
Bank, as Trustee, as supplemented by certain indentures supplemental thereto,
executed and delivered to the Trustee; and this bond is one of a series of such
bonds, designated "Northern Illinois Gas Company First Mortgage Bonds,       %
Series due             " (herein called "bonds of this Series"), the issuance of
which is provided for by a Supplemental Indenture dated             (hereinafter
called the "Supplemental Indenture"), executed and delivered by the Company to
the Trustee. The term "Indenture", as hereinafter used, means said Indenture
dated as of January 1, 1954, and all indentures supplemental thereto from time
to time in effect. Reference is made to the Indenture for a description of the
property mortgaged and pledged, the nature and extent of the security, the
rights of the holders and registered owners of said bonds, of the Company and of
the Trustee in respect of the security, and the terms and conditions governing
the issuance and security of said bonds.
 
     With the consent of the Company and to the extent permitted by and as
provided in the Indenture, modifications or alterations of the Indenture or of
any supplemental indenture and of the rights and obligations of the Company and
of the holders and registered owners of the bonds may be made, and compliance
with any provision of the Indenture or of any supplemental indenture may be
waived, by the affirmative vote of the holders and registered owners of not less
than sixty-six and two-thirds per centum (66 2/3%) in principal amount of the
bonds then outstanding under the Indenture, and by the affirmative vote of the
holders and
 
                                        2
<PAGE>   4
 
registered owners of not less than sixty-six and two-thirds per centum (66 2/3%)
in principal amount of the bonds of any series then outstanding under the
Indenture and affected by such modification or alteration, in case one or more
but less than all of the series of bonds then outstanding under the Indenture
are so affected, but in any case excluding bonds disqualified from voting by
reason of the Company's interest therein as provided in the Indenture; subject,
however, to the condition, among other conditions stated in the Indenture, that
no such modification or alteration shall be made which, among other things, will
permit the extension of the time or times of payment of the principal of or the
interest or the premium, if any, on this bond, or the reduction in the principal
amount hereof or in the rate of interest or the amount of any premium hereon, or
any other modification in the terms of payment of such principal, interest or
premium, which terms of payment are unconditional, or, otherwise than as
permitted by the Indenture, the creation of any lien ranking prior to or on a
parity with the lien of the Indenture with respect to any of the mortgaged
property, all as more fully provided in the Indenture.
 
                      (Alternative Redemption Provisions)
 
     [The bonds of this Series may not be called for redemption by the Company
prior to                     . On                     and thereafter until
maturity on                     , upon the notice hereinafter stated and in the
manner and with the effect provided in the Indenture, the bonds of this Series
are redeemable at the option of the Company, as a whole at any time or in part
from time to time prior to the maturity thereof, at the applicable redemption
price (expressed as a percentage of principal amount) set forth below under
"General Redemption Prices," plus accrued and unpaid interest to the redemption
date:
 
                           General Redemption Prices
 
              ----------------------------------------------------
                       If Redeemed During Twelve Months'
                       Period Beginning                :
 
              ----------------------------------------------------
 
     (The years and the percentages of principal amount set forth under "General
     Redemption Prices" in Section 4 of Article I of this Supplemental Indenture
     are to be inserted here.)]
 
     [The bonds of this Series may not be called for redemption by the Company
prior to                     . On                     and thereafter until
maturity on                     , the bonds of this Series, upon the notice
hereinafter stated and in the manner and with the effect provided in the
Indenture, will be redeemable at the option of the Company, as a whole at any
time or in part from time to time, at 100% of the principal amount thereof, plus
accrued and unpaid interest to the redemption date.]
 
     Notice of each redemption shall be mailed to all registered owners not less
than thirty nor more than forty-five days before the redemption date.
 
     In case of certain completed defaults specified in the Indenture, the
principal of this bond may be declared or may become due and payable in the
manner and with the effect provided in the Indenture.
 
     No recourse shall be had for the payment of the principal of or the
interest or the premium, if any, on this bond, or for any claim based hereon, or
otherwise in respect hereof or of the Indenture, to or against any incorporator,
stockholder, officer or director, past, present or future, of the Company or of
any predecessor or successor corporation, either directly or through the Company
or such predecessor or successor corporation, under any constitution or statute
or rule of law, or by the enforcement of any assessment or penalty, or
otherwise, all such liability of incorporators, stockholders, directors and
officers being waived and released by the registered owner hereof by the
acceptance of this bond and being likewise waived and released by the terms of
the Indenture, all as more fully provided therein.
 
     This bond is transferable by the registered owner hereof, in person or by
duly authorized attorney, at the office or agency of the Company in the City of
Chicago, State of Illinois, or, at the option of the registered owner, at the
office or agency of the Company in the Borough of Manhattan, The City and State
of New York, upon surrender and cancellation of this bond; and thereupon a new
registered bond or bonds without coupons of the same aggregate principal amount
and series will, upon the payment of any transfer tax or taxes payable,
 
                                        3
<PAGE>   5
 
be issued to the transferee in exchange herefor. The Company shall not be
required to exchange or transfer this bond if this bond or a portion hereof has
been selected for redemption.
 
                               (END OF BOND FORM)
and
 
     WHEREAS, all acts and things necessary to make this Supplemental Indenture,
when duly executed and delivered, a valid, binding and legal instrument in
accordance with its terms and for the purposes herein expressed, have been done
and performed, and the execution and delivery of this Supplemental Indenture
have in all respects been duly authorized;
 
     NOW, THEREFORE, in consideration of the premises and of the sum of one
dollar paid by the Trustee to the Company, and for other good and valuable
considerations, the receipt of which is hereby acknowledged, for the purpose of
securing the due and punctual payment of the principal of and the interest and
premium, if any, on all bonds which shall be issued under the Indenture, and for
the purpose of securing the faithful performance and observance of all the
covenants and conditions set forth in the Indenture and in all indentures
supplemental thereto, the Company by these presents does grant, bargain, sell,
transfer, assign, pledge, mortgage, warrant and convey unto Harris Trust and
Savings Bank, as Trustee, and its successor or successors in the trust hereby
created, all property, real and personal (other than property expressly excepted
from the lien and operation of the Indenture), which, at the actual date of
execution and delivery of this Supplemental Indenture, is solely used or held
for use in the operation by the Company of its gas utility system and in the
conduct of its gas utility business and all property, real and personal, used or
useful in the gas utility business (other than property expressly excepted from
the lien and operation of the Indenture) acquired by the Company after the
actual date of execution and delivery of this Supplemental Indenture or (subject
to the provisions of Section 16.03 of the Indenture) by any successor
corporation after such execution and delivery, and it is further agreed by and
between the Company and the Trustee as follows:
 
                                   ARTICLE I
 
                              BONDS OF THIS SERIES
 
     SECTION 1. The bonds of this Series shall, as hereinbefore recited, be
designated as the Company's "First Mortgage Bonds,      % Series due
            ." The bonds of this Series which may be issued and outstanding
shall not exceed $   ,000,000 in aggregate principal amount, exclusive of bonds
of such series authenticated and delivered pursuant to the provisions of Section
4.12 of the Indenture.
 
     SECTION 2. The bonds of this Series shall be registered bonds without
coupons, and the form of such bonds, and of the Trustee's certificate of
authentication to be endorsed on all bonds of this Series, shall be
substantially as hereinbefore recited, respectively.
 
     SECTION 3. The bonds of this Series shall be issued in the denomination of
$1,000 each and in such multiple or multiples thereof as shall be determined and
authorized by the Board of Directors of the Company or by any officer or
officers of the Company authorized by the Board of Directors to make such
determination, the authorization of the denomination of any bond to be
conclusively evidenced by the execution thereof on behalf of the Company. The
bonds of this Series shall be numbered, RU-1 and consecutively upwards, or in
such other appropriate manner as shall be determined and authorized by the Board
of Directors of the Company.
 
     All bonds of this Series shall be dated             , except that each bond
issued on or after the first payment of interest thereon shall be dated as of
the date of the interest payment date thereof to which interest shall have been
paid on the bonds of such series next preceding the date of issue, unless issued
on an interest payment date to which interest shall have been so paid, in which
event such bonds shall be dated as of the date of issue; provided, however, that
bonds issued on or after             and before the next succeeding
or on or after             and before the next succeeding             shall be
dated the next succeeding interest payment date if interest shall have been paid
to such date. All bonds of this Series shall mature             , and shall bear
interest at the rate of       % per annum until the principal thereof shall
 
                                        4
<PAGE>   6
 
be paid. Such interest shall be calculated on the basis of a 360-day year
consisting of twelve 30-day months and shall be payable semiannually on the
       day of        and the        day of        in each year. So long as there
is no existing default in the payment of interest on the bonds of this Series,
such interest shall be payable to the person in whose name each such bond is
registered on the        or the        (whether or not a business day), as the
case may be, next preceding the respective interest payment dates; provided,
however, if and to the extent that the Company shall default in the payment of
interest due on such interest payment date, such defaulted interest shall be
paid to the person in whose name each such bond is registered on the record date
fixed, in advance, by the Company for the payment of such defaulted interest.
 
     The principal of and interest and premium, if any, on the bonds of this
Series shall be payable in any coin or currency of the United States of America
which at the time of payment is legal tender for the payment of public and
private debts, and shall be payable at the office or agency of the Company in
the City of Chicago, State of Illinois, or, at the option of the registered
owner, at the office or agency of the Company in the Borough of Manhattan, The
City and State of New York. Any installment of interest on the bonds may, at the
Company's option, be paid by mailing checks for such interest payable to or upon
the written order of the person entitled thereto to the address of such person
as it appears on the registration books. The bonds of this Series shall be
registrable, transferable and exchangeable in the manner provided in Sections
4.08 and 4.09 of the Indenture, at either of such offices or agencies.
 
                      (Alternative Redemption Provisions)
 
     [SECTION 4. The bonds of this Series may not be called for redemption by
the Company prior to             . On             and thereafter until maturity
on                     , upon the mailing of notice and in the manner provided
in Section 7.01 of the Indenture (except that no published notice shall be
required for bonds of this Series), and with the effect provided in Section 7.02
thereof, the bonds of this Series shall be redeemable at the option of the
Company, as a whole at any time or in part from time to time prior to the
maturity thereof, at the applicable redemption price (expressed as a percentage
of principal amount) set forth below under "General Redemption Prices," plus
accrued and unpaid interest to the redemption date:
 
                           General Redemption Prices
 
              ----------------------------------------------------
                       If Redeemed During Twelve Months'
                       Period Beginning                :
 
              ----------------------------------------------------
 
<TABLE>
<CAPTION>
                YEAR   PERCENTAGE   YEAR   PERCENTAGE
                ----   ----------   ----   ----------
                <S>    <C>          <C>    <C>
                             %                  %]
</TABLE>
 
     [SECTION 4. The bonds of this Series may not be called for redemption by
the Company prior to                     . On                     and thereafter
until maturity on                     , the bonds of this Series, upon the
mailing of notice and in the manner provided in Section 7.01 of the Indenture
(except that no published notice shall be required for the bonds of this
Series), and with the effect provided in Section 7.02 thereof, will be
redeemable at the option of the Company, as a whole at any time or in part from
time to time, at 100% of the principal amount thereof, plus accrued and unpaid
interest to the redemption date.]
 
     SECTION 5. No sinking fund is to be provided for the bonds of this Series.
 
                                        5
<PAGE>   7
 
                                   ARTICLE II
 
                            MISCELLANEOUS PROVISIONS
 
     SECTION 1. This Supplemental Indenture is executed by the Company and the
Trustee pursuant to provisions of Section 4.02 of the Indenture and the terms
and conditions hereof shall be deemed to be a part of the terms and conditions
of the Indenture for any and all purposes. The Indenture, as heretofore
supplemented and as supplemented by this Supplemental Indenture, is in all
respects ratified and confirmed.
 
     SECTION 2. This Supplemental Indenture shall bind and, subject to the
provisions of Article XVI of the Indenture, inure to the benefit of the
respective successors and assigns of the parties hereto.
 
     SECTION 3. Although this Supplemental Indenture is dated
                    , it shall be effective only from and after the actual time
of its execution and delivery by the Company and the Trustee on the date
indicated by their respective acknowledgments hereto annexed.
 
     SECTION 4. This Supplemental Indenture may be simultaneously executed in
any number of counterparts, and all such counterparts executed and delivered,
each as an original, shall constitute but one and the same instrument.
 
     IN WITNESS WHEREOF, Northern Illinois Gas Company has caused this
Supplemental Indenture to be executed in its name by its President, a Vice
President, or Treasurer, and its corporate seal to be hereunto affixed and
attested by its Secretary or its Assistant Secretary, and Harris Trust and
Savings Bank, as Trustee under the Indenture, has caused this Supplemental
Indenture to be executed in its name by one of its Vice Presidents, and its seal
to be hereunto affixed and attested by one of its Assistant Secretaries, all as
of the day and year first above written.
 
NORTHERN ILLINOIS GAS COMPANY
 
BY
- --------------------------------------
      Vice President and Treasurer
                                          ATTEST:
 
                                          --------------------------------------
                                                   Assistant Secretary
HARRIS TRUST AND SAVINGS BANK,
  as Trustee
 
By
- --------------------------------------
             Vice President
                                          ATTEST:
 
                                          --------------------------------------
                                                   Assistant Secretary
 
                                        6
<PAGE>   8
 
STATE OF ILLINOIS               SS:
COUNTY OF
 
     I,                , a Notary Public in in the State aforesaid, DO HEREBY
CERTIFY that                , Vice President and Treasurer of Northern Illinois
Gas Company, an Illinois corporation, one of the parties described in and which
executed the foregoing instrument, and                , Assistant Secretary of
said corporation, who are both personally known to me to be the same persons
whose names are subscribed to the foregoing instrument as such Vice President
and Treasurer and Assistant Secretary, respectively, and who are both personally
known to me to be Vice President and Treasurer and the Assistant Secretary,
respectively, of said corporation, appeared before me this day in person and
severally acknowledged that they signed, sealed, executed and delivered said
instrument as their free and voluntary act as such Vice President an Treasurer
and Assistant Secretary, respectively, of said corporation, and as the free and
voluntary act of said corporation, for the uses and purposes therein set forth.
 
     GIVEN under my hand and notarial seal this           day of           A.D.
 
                                          --------------------------------------
                                                      Notary Public
 
My Commission expires
 
                                        7
<PAGE>   9
 
STATE OF ILLINOIS               SS:
COUNTY OF
 
     I,                , a Notary Public in and for said County, in the State
aforesaid, DO HEREBY CERTIFY that                , Vice President of Harris
Trust and Savings Bank, an Illinois banking corporation, one of the parties
described in and which executed the foregoing instrument, and                ,
an Assistant Secretary of said banking association, who are both personally
known to me to be the same persons whose names are subscribed to the foregoing
instrument as such Vice President and Assistant Secretary, respectively, and who
are both personally known to me to be a Vice President and an Assistant
Secretary, respectively, of said banking association, appeared before me this
day in person and severally acknowledged that they signed, sealed, executed and
delivered said instrument as their free and voluntary act as such Vice President
and Assistant Secretary, respectively, of said banking association, and as the
free and voluntary act of said banking association, for the uses and purposes
therein set forth.
 
     GIVEN under my hand and notarial seal this           day of           A.D.
 
                                          --------------------------------------
                                                      Notary Public
 
My Commission expires
 
                                        8
<PAGE>   10
 
                                 RECORDING DATA
 
     This Supplemental Indenture was recorded on               ,      , in the
office of the Recorder of Deeds in certain counties in the State of Illinois, as
follows:
 
<TABLE>
<CAPTION>
    COUNTY        BOOK    PAGE  DOCUMENT NO.
    ------        ----    ----  ------------
<S>              <C>      <C>   <C>
Adams..........
Boone..........
Bureau.........
Carroll........
Champaign......
Cook...........
DeKalb.........
DeWitt.........
DuPage.........
Ford...........
Grundy.........
Hancock........
Henderson......
Henry..........
Iroquois.......
Jo Daviess.....
Kane...........
Kankakee.......
Kendall........
Lake...........
La Salle.......
Lee............
Livingston.....
McHenry........
McLean.........
Mercer.........
Ogle...........
Platt..........
Pike...........
Rock Island....
Stephenson.....
Tazewell.......
Vermillon......
Whiteside......
Will...........
Winnebago......
Woodford.......
</TABLE>
 
                                        9

<PAGE>   1






                                                                    EXHIBIT 5.01



                               December 17, 1997



Northern Illinois Gas Company
1844 Ferry Road
Naperville, Illinois  60563-9600

Ladies and Gentlemen:

     We are representing Northern Illinois Gas Company (the "Company"), in
connection with the proposed sale of up to $175,000,000 principal amount of the
Company's First Mortgage Bonds (the "Bonds").  The Bonds are to be issued under
the Company's Indenture (the "Indenture") dated as of January 1, 1954, as
supplemented by one or more Supplemental Indentures to be entered into prior to
the issuance of the Bonds, with certain terms of the Bonds and the Supplemental
Indentures to be approved by or pursuant to resolutions of the Board of
Directors of the Company or a committee thereof as part of the corporate action
taken and to be taken (the "Corporate Proceedings") relating to the issuance of
the Bonds.  We have examined or are otherwise familiar with the Articles of
Incorporation of the Company, as amended, the By-Laws of the Company, as
amended, the Company's Registration Statement pursuant to which the Bonds are
to be registered under the Securities Act of 1933, the Corporate Proceedings
and such other documents, records, and instruments as we have deemed necessary
for the purposes of this opinion.

     Based on the foregoing, we are of the opinion that, upon the execution and
delivery of each Supplemental Indenture, the completion of the Corporate
Proceedings and the authentication, sale and delivery of the Bonds issuable
thereunder, such Supplemental Indenture will become a valid and binding
instrument and the Bonds issuable thereunder will be legal, valid and binding
obligations of the Company, entitled to the benefits of the Indenture and such
Supplemental Indenture, including such terms as are established pursuant to 
the Corporate Proceedings, in accordance with the respective terms thereof, 
except as enforcement of provisions of the Indenture may be limited by 
bankruptcy or other laws of general application affecting the enforcement of 
creditors' rights and by general equity principles.




<PAGE>   2

Northern Illinois Gas Company
December 17, 1997
Page 2



     We hereby consent to the filing of this opinion as Exhibit 5.01 to the
Company's registration statement and to being named in the prospectus under the
caption "Legal Opinions" with respect to the matters stated therein.

                                                       Very truly yours,



                                                       MAYER, BROWN & PLATT




<PAGE>   1
                                                              EXHIBIT 23.01
     
     
     
     
     
                 CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS
     
     
As independent public accountants, we hereby consent to the incorporation by 
reference in this registration statement of our report dated January 28, 1997, 
included in Northern Illinois Gas Company's Form 10-K for the year ended 
December 31, 1996, and to all references to our firm included in this 
registration statement.
     
     
     
     
                                                        ARTHUR ANDERSEN LLP
     
Chicago, Illinois
December 18, 1997

<PAGE>   1
                                                           EXHIBIT 24.01
     
     
     
     
     
     
     
     
                             POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                      ROBERT M. BEAVERS, JR.       
                                      Robert M. Beavers, Jr.
     
     

<PAGE>   2

     
     
                              POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                         BRUCE P. BICKNER          
                                         Bruce P. Bickner
     
     

<PAGE>   3

     
     
                              POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                       JOHN H. BIRDSALL, III       
                                       John H. Birdsall, III
     
     

<PAGE>   4

     
     
                              POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                           JOHN E. JONES           
                                           John E. Jones
     

<PAGE>   5

     
     
     
                              POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                         DENNIS J. KELLER          
                                         Dennis J. Keller
     

<PAGE>   6

     
     
                              POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                         CHARLES S. LOCKE          
                                         Charles S. Locke
     

<PAGE>   7

     
     
     
                              POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                        SIDNEY R. PETERSEN         
                                        Sidney R. Petersen
     

<PAGE>   8

     
     
     
                              POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                          DANIEL R. TOLL           
                                          Daniel R. Toll
     
     

<PAGE>   9

     
     
                             POWER OF ATTORNEY
     
     
KNOW ALL MEN BY THESE PRESENTS:
     
          That the undersigned, a Director, Officer or Director and Officer
of Northern Illinois Gas Company, an Illinois corporation d/b/a Nicor Gas 
Company, does hereby constitute and appoint D. W. LOHRENTZ, D. L. CYRANOSKI, 
and G. M. BEHRENS, and each of them, the undersigned's true and lawful 
attorneys and agents, each with full power and authority (acting alone and 
without the others) to execute in the name and on behalf of the undersigned 
as such Director, Officer or Director and Officer, a shelf Registration 
Statement (and such amendment or amendments thereto as may be necessary) 
under the Securities Act of 1933, with respect to and not to exceed 
$175,000,000 principal amount of the company's First Mortgage Bonds, hereby 
granting to such attorneys and agents, and each of them, full power of 
substitution and revocation in the premises; and hereby ratifying and 
confirming all that such attorneys and agents, or any of them, may do or 
cause to be done by virtue of these presents.
     
          IN WITNESS WHEREOF, I have hereunto signed this Power of Attorney
this 11th day of December, 1997.
     
     
                                         PATRICIA A. WIER          
                                         Patricia A. Wier
     
     
     
     

<PAGE>   1
                                                        EXHIBIT 25.01


                     SECURITIES AND EXCHANGE COMMISSION
                           Washington, D.C.  20549


                                  FORM T-1


                          Statement of Eligibility
                    Under the Trust Indenture Act of 1939
                    of a Corporation Designated to Act as
                                   Trustee


                    Check if an Application to Determine
                Eligibility of a Trustee Pursuant to Section
                          305(b)(2) _______________


                        HARRIS TRUST AND SAVINGS BANK
                              (Name of Trustee)



        Illinois                                         23-1614034
                                                      (I.R.S. Employer
   (State of Incorporation)                          Identification No.)


               111 West Monroe Street, Chicago, Illinois 60603
                  (Address of principal executive offices)


              Daniel G. Donovan, Harris Trust and Savings Bank,
              111 West Monroe Street, Chicago, Illinois, 60603
                                312-461-2908
         (Name, address and telephone number for agent for service)


                        NORTHERN ILLINOIS GAS COMPANY
                              (Name of Obligor)


        Illinois                                         36-2863847
                                                      (I.R.S. Employer
   (State of Incorporation)                          Identification No.)


                              1844 Ferry Road,
                      Naperville, Illinois  60563-9600
                  (Address of principal executive offices)

                            First Mortgage Bonds
                       (Title of indenture securities)

<PAGE>   2


1.  GENERAL INFORMATION.  Furnish the following information as to the Trustee:

    (a)  Name and address of each examining or supervising authority to which
         it is subject.

            Commissioner of Banks and Trust Companies, State of Illinois,
            Springfield, Illinois; Chicago Clearing House Association, 164 West
            Jackson Boulevard, Chicago, Illinois; Federal Deposit Insurance
            Corporation, Washington, D.C.; The Board of Governors of the Federal
            Reserve System,Washington, D.C.

    (b)  Whether it is authorized to exercise corporate trust powers.

         Harris Trust and Savings Bank is authorized to exercise corporate
         trust powers.

2.  AFFILIATIONS WITH OBLIGOR.  If the Obligor is an affiliate of the   
    Trustee, describe each such affiliation.

         The Obligor is not an affiliate of the Trustee.

3. thru 15.

         NO RESPONSE NECESSARY

16. LIST OF EXHIBITS.

     1.  A copy of the articles of association of the Trustee as now in
         effect which includes the authority of the trustee to commence
         business and to exercise corporate trust powers.

         A copy of the Certificate of Merger dated April 1, 1972 between Harris
         Trust and Savings Bank, HTS Bank and Harris Bankcorp, Inc. which
         constitutes the articles of association of the Trustee as now in effect
         and includes the authority of the Trustee to commence business and to
         exercise corporate trust powers was filed in connection with the
         Registration Statement of Louisville Gas and Electric Company, File No.
         2-44295, and is incorporated herein by reference.

     2.  A copy of the existing by-laws of the Trustee.

         A copy of the existing by-laws of the Trustee was filed in 
         connection  with  the Registration Statement of  Commercial
         Federal Corporation, File No. 333-20711, and is incorporated herein by
         reference.

     3.  The consents of the Trustee required by Section 321(b) of the
         Act.

                (included as Exhibit A on page 2 of this statement)

     4.  A copy of the latest report of condition of the Trustee
         published pursuant to law or the requirements of its supervising or
         examining authority.

                (included as Exhibit B on page 3 of this statement)



<PAGE>   3



                                   SIGNATURE


Pursuant to the requirements of the Trust Indenture Act of 1939, the Trustee,
HARRIS TRUST AND SAVINGS BANK, a corporation organized and existing under the
laws of the State of Illinois, has duly caused this statement of eligibility to
be signed on its behalf by the undersigned, thereunto duly authorized, all in
the City of Chicago, and State of Illinois, on the 16th day of December 1997.

HARRIS TRUST AND SAVINGS BANK


By:   /s/ DGDonovan
   ---------------------------
      D. G. Donovan
      Assistant Vice President


EXHIBIT A

The consents of the Trustee required by Section 321(b) of the Act.

Harris Trust and Savings Bank, as the Trustee herein named, hereby consents
that reports of examinations of said trustee by Federal and State authorities
may be furnished by such authorities to the Securities and Exchange Commission
upon request therefor.

HARRIS TRUST AND SAVINGS BANK

By:   /s/ DGDonovan
- ------------------------------
      D.G. Donovan
      Assistant Vice President




                                      2
<PAGE>   4


                                                                EXHIBIT B


Attached is a true and correct copy of the statement of condition of Harris
Trust and Savings Bank as of September 30, 1997, as published in accordance
with a call made by the State Banking Authority and by the Federal Reserve
Bank of the Seventh Reserve District.



           [ HARRIS BANK LOGO]
                
                                  HARRIS BANK

                         Harris Trust and Savings Bank
                             111 West Monroe Street
                            Chicago, Illinois  60603


of Chicago, Illinois, And Foreign and Domestic Subsidiaries, at the close of
business on September 30, 1997, a state banking institution organized and
operating under the banking laws of this State and a member of the Federal
Reserve System. Published in accordance with a call made by the Commissioner
of Banks and Trust Companies of the State of Illinois and by the Federal
Reserve Bank of this District.                                                 



                         Bank's Transit Number 71000288

<TABLE>
<CAPTION>

                        
                                                                                                THOUSANDS
                                            ASSETS                                              OF DOLLARS
<S>                                                                                     <C>             <C>
Cash and balances due from depository institutions:
          Non-interest bearing balances and currency and coin........................                   $1,188,709
          Interest bearing  balances.................................................                     $550,173
Securities:..........................................................................
a.  Held-to-maturity securities                                                                                  $0
b.  Available-for-sale securities                                                                       $3,685,983
Federal funds sold and securities purchased under agreements to resell                                    $396,400
Loans and lease financing receivables:
          Loans and leases, net of unearned income. .................................   $8,401,048
          LESS:  Allowance for loan and lease losses ................................     $107,180
                                                                                      -------------     
          Loans and leases, net of unearned income, allowance, and reserve
          (item 4.a minus 4.b).......................................................                   $8,293,868
Assets held in trading accounts......................................................                      $98,368
Premises and fixed assets (including capitalized leases).............................                     $213,612
Other real estate owned..............................................................                         $778
Investments in unconsolidated subsidiaries and associated companies..................                          $86
Customer's liability to this bank on acceptances outstanding.........................                      $41,205
Intangible assets....................................................................                     $283,839
Other assets.........................................................................                     $603,886
                                                                                                ------------------

TOTAL ASSETS                                                                                           $15,356,907
                                                                                                ===================

</TABLE>
                                       3
<PAGE>   5

<TABLE>
<S>                                                                                             <C>             <C>
                                                  LIABILITIES
Deposits:
In domestic offices......................................................................                       $8,374,055
     Non-interest bearing................................................................       $2,770,029
     Interest bearing....................................................................       $5,604,026
In foreign offices, Edge and Agreement subsidiaries, and IBF's...........................                       $1,991,659
     Non-interest bearing................................................................          $27,364
     Interest bearing....................................................................       $1,964,295
Federal funds purchased and securities sold under agreements to repurchase in 
     domestic offices of the bank and of its Edge and Agreement subsidiaries, 
     and in IBF's:
   Federal funds purchased & securites sold under agreements to repurchase...............                       $2,549,328
Trading Liabilities                                                                                                 62,186
Other borrowed money:....................................................................
a.  With remaining maturity of one year or less                                                                   $630,911
b.  With remaining maturity of more than one year                                                                       $0
Bank's liability on acceptances executed and outstanding                                                           $41,205
Subordinated notes and debentures........................................................                         $325,000
Other liabilities........................................................................                         $132,188
                                                                                                --------------------------
TOTAL LIABILITIES                                                                                              $14,106,532
                                                                                                ==========================


                                                EQUITY CAPITAL
Common stock.............................................................................                         $100,000
Surplus..................................................................................                         $600,853
a.  Undivided profits and capital reserves...............................................                         $553,257
b.  Net unrealized holding gains (losses) on available-for-sale securities                                         ($3,735)
                                                                                                --------------------------
TOTAL EQUITY CAPITAL                                                                                            $1,250,375
                                                                                                ==========================
Total liabilities, limited-life preferred stock, and equity capital......................                      $15,356,907
                                                                                                ==========================

</TABLE>
     

        I, Pamela Piarowski, Vice President of the above-named bank, do hereby
declare that this Report of Condition has been prepared in conformance with
the instructions issued by the Board of Governors of the Federal Reserve
System and is true to the best of my knowledge and belief.

                               PAMELA PIAROWSKI
                                   10/29/97

     We, the undersigned directors, attest to the correctness of this Report
of Condition and declare that it has been examined by us and, to the best of
our knowledge and belief, has been prepared in conformance with the
instructions issued by the Board of Governors of the Federal Reserve System
and the Commissioner of Banks and Trust Companies of the State of Illinois and
is true and correct.

          EDWARD W. LYMAN,
          ALAN G. McNALLY,
          JAMES J. GLASSER
                                                                      Directors.
                                      4








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