NATURAL MICROSYSTEMS CORP
8-A12G, 1999-01-19
TELEPHONE & TELEGRAPH APPARATUS
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<PAGE>
 
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549

                                   FORM 8-A

         FOR REGISTRATION OF CERTAIN CLASSES OF SECURITIES PURSUANT TO
          SECTION 12(b) OR (g) OF THE SECURITIES EXCHANGE ACT OF 1934


                       NATURAL MICROSYSTEMS CORPORATION
                       --------------------------------
            (Exact Name of Registrant as Specified in Its Charter)


               Delaware                                  04-2814586
               --------                                  ---------- 
(State of Incorporation or Organization)               (IRS Employer
                                                     Identification no.)

           100 Crossing Boulevard, Framingham, Massachusetts 01702
           ------------------------------------------------- -----
           (Address of Principal Executive Offices)       (Zip Code)

If this form relates to       If this form relates to the
the registration of a         registration of a class of
class of securities           securities pursuant to Section
pursuant to Section 12(b)     12(g) of the Exchange Act
of the Exchange Act           and is effective pursuant to
and is effective pursuant     General Instruction A.(d),
to General Instruction        please check the following
A.(c), please check the       box.   [x]
following box. [ ]
 
 
Securities to be registered pursuant to Section 12(b) of the Act:

                                         Name of Each Exchange
Title of Each Class                      on Which Each Class is
to be so Registered                         to be Registered
- -------------------                         ----------------

      None                                        None



Securities to be registered pursuant to Section 12(g) of the Act:

                SERIES A JUNIOR PREFERRED STOCK PURCHASE RIGHTS
                -----------------------------------------------
                               (Title of Class)
                                        
<PAGE>
 
Item 1.   Description of Registrant's Securities to be Registered
          -------------------------------------------------------

     On December 23, 1998, the Board of Directors (the "Board") of Natural
MicroSystems Corporation (the "Company") declared a dividend of one purchase
right ("Right") for every outstanding share of the Company's common stock, $.01
par value (the "Common Stock").  The Rights will be distributed to holders of
record of the Common Stock as of the close of business on January 4, 1999 (the
"Dividend Record Date").  The terms of the Rights are set forth in a Rights
Agreement dated as of December 23, 1998 (the "Rights Agreement") between the
Company and State Street Bank and Trust Company (the "Rights Agent").  The
Rights Agreement provides for the issuance of one Right for every share of
Common Stock issued and outstanding on the Dividend Record Date and for each
share of Common Stock which is issued or sold after that date and prior to the
Distribution Date (as defined below).

     Each Right entitles the holder to purchase from the Company one one-
thousandth of a share of Series A Junior Preferred Stock, $.01 par value, of the
Company (the "Junior Preferred Stock"), at a price of $60 per one one-thousandth
of a share, subject to adjustments in certain events.  The Rights will expire on
December 23, 2008 (the "Expiration Date"), or upon the earlier redemption of the
Rights, and are not exercisable until the Distribution Date.

     No separate Rights certificates will be issued at the present time.  Until
the Distribution Date (or earlier redemption or expiration of the Rights), (i)
the Rights will be evidenced by the outstanding Common Stock certificates and
will be transferred with and only with the Common Stock certificates, (ii) new
Common Stock certificates issued after the Dividend Record Date upon transfer or
new issuance of the Common Stock will contain a notation incorporating the
Rights Agreement by reference and (iii) the surrender for transfer of any Common
Stock certificate will also constitute the transfer of the Rights associated
with the Common Stock represented by such certificate.

     The Rights will separate from the Common Stock on the Distribution Date.
Unless otherwise determined by a majority of the Continuing Directors (as
defined below) then in office, the Distribution Date (the "Distribution Date")
will occur on the earlier of (i) the tenth day following the date of a public
announcement that a person, together with its affiliates and associates, except
as described below, has acquired or obtained the rights to acquire beneficial
ownership of 15% or more of the outstanding shares of Common Stock
(collectively, an "Acquiring Person") (such date is referred to herein as the
"Shares Acquisition Date") or (ii) the tenth day following commencement of a
tender offer or exchange offer that would result in any person, together with
its affiliates and associates, owning 30% or more of the outstanding Common
Stock.  After the Distribution 

                                       2
<PAGE>
 
Date, separate certificates evidencing the Rights ("Rights Certificates") will
be mailed to holders of record of the Common Stock as of the close of business
on the Distribution Date and thereafter such separate Rights Certificates alone
will evidence the Rights. The Board, by action of the Continuing Directors, may
delay the distribution of the Certificates. The term "Continuing Directors"
means (i) any member of the Company's Board of Directors who is not an Acquiring
Person, or an affiliate, associate or representative of an Acquiring Person, or
(ii) any person who subsequently becomes a member of the Board, who is not an
Acquiring Person or an affiliate, associate or representative of an Acquiring
Person, if such person's nomination for election or election to the Board is
recommended or approved by a majority of Continuing Directors.

     If, at any time after January 4, 1999, any person or group of affiliated or
associated persons (other than the Company and its affiliates) shall become an
Acquiring Person, each holder of a Right will have the right to receive shares
of Common Stock (or, in certain circumstances, cash, property or other
securities of the Company) having a market value of two times the exercise price
of the Right.  Following the occurrence of any such event, any Rights that are,
or (under certain circumstances specified in the Rights Agreement) were,
beneficially owned by any Acquiring Person shall immediately become null and
void.  Also, if the Company were acquired in a merger or other business
combination, or if more than 50% of its assets or earning power were sold, each
holder of a Right would have the right to exercise such Right and thereby
receive common stock of the acquiring company with a market value of two times
the exercise price of the Right.

     The Board may, at its option, at any time after any person becomes an
Acquiring Person, exchange all or part of the then outstanding and exercisable
Rights for shares of Common Stock at an exchange ratio of one share of Common
Stock per Right, appropriately adjusted to reflect any stock split, stock
dividend or similar transaction occurring after January 4, 1999 (as the same may
be adjusted, the "Exchange Ratio").  The Board however, may not effect an
exchange at any time after any person (other than (i) the Company, (ii) any
subsidiary of the Company, (iii) any employee benefit plan of the Company or of
any subsidiary of the Company or (iv) any entity holding Common Stock for or
pursuant to the terms of any such plan), together with all affiliates of such
person, becomes the beneficial owner of 50% or more of the Common Stock then
outstanding.  Immediately upon the action of the Board ordering the exchange of
any Rights and without any further action and without any notice, the right to
exercise such Rights will terminate and the only right thereafter of a holder of
such Rights will be to receive that number of shares of Common Stock equal to
the number of such Rights held by the holder multiplied by the Exchange Ratio.

                                       3
<PAGE>
 
     The exercise price of the Rights, and the number of one one-thousandths of
a share of Junior Preferred Stock or other securities or property issuable upon
exercise of the Rights, are subject to adjustment from time to time to prevent
dilution (i) in the event of a stock dividend on, or a subdivision, combination
or reclassification of, the Junior Preferred Stock, (ii) upon the grant to
holders of the Junior Preferred Stock of certain rights or warrants to subscribe
for shares of the Junior Preferred Stock or certain convertible securities at
less than the current market price of the Junior Preferred Stock, or (iii) upon
the distribution to holders of the Junior Preferred Stock of evidences of
indebtedness or assets (excluding cash dividends paid out of the earnings or
retained earnings of the Company and certain other distributions) or of
subscription rights, or warrants (other than those referred to above).

     At any time prior to the tenth day (or such later date as may be determined
by a majority of the Continuing Directors) after the Shares Acquisition Date
(the "Expiration Date"), the Company, by a majority vote of the Continuing
Directors, may redeem the Rights at a redemption price of $.01 per Right,
subject to adjustment in certain events (as the same may be adjusted, the
"Redemption Price").  Immediately upon the action of the Continuing Directors
electing to redeem the rights, the right to exercise the Rights will terminate,
and the only right of the holders of Rights will be to receive the Redemption
Price.

     Until a Right is exercised, the holder thereof, as such, will have no
rights as a stockholder of the Company, including, without limitation, the right
to vote or to receive dividends.

     The Rights Agreement may be amended by the Board at any time prior to the
Distribution Date without the approval of the holders of the Rights.  From and
after the Distribution Date, the Rights Agreement may be amended by the Board
without the approval of the holders of the Rights in order to cure any
ambiguity, to correct any defective or inconsistent provisions, to change any
time period for redemption or any other time period under the Rights Agreement
or to make any other changes that do not adversely affect the interests of the
holders of the Rights (other than any Acquiring Person or its affiliates and
associates or their transferees).

     A copy of the Rights Agreement, dated as of December 23, 1998, between the
Company and State Street Bank and Trust Company, as Rights Agent, specifying the
terms of the Rights (including as exhibits the form of Rights Certificate and
the Summary of Rights) is attached hereto as an exhibit.  The foregoing
description of the Rights does not purport to be complete and is qualified in
its entirety by reference to the Rights Agreement, which is incorporated herein
by reference.

                                       4
<PAGE>
 
Item 2.   Exhibits
          --------

1.   Rights Agreement, dated as of December 23, 1998 (the "Rights Agreement"),
     between Natural MicroSystems Corporation and State Street Bank and Trust
     Company, as Rights Agent.

2.   Form of Certificate of Designations, Rights, Preferences and Privileges of
     the Series A Junior Preferred Stock, par value $.01 per share, of the
     Company (attached as Exhibit A to the Rights Agreement).

3.   Form of Rights Certificate (attached as Exhibit B to the Rights Agreement).
     Pursuant to the Rights Agreement, printed Rights Certificates will not be
     mailed until the Distribution Date (as defined in the Rights Agreement).

4.   Summary of Rights (attached as Exhibit C to the Rights Agreement).


                                   SIGNATURE

     Pursuant to the requirements of Section 12 of the Securities Exchange Act
of 1934, the Registrant has duly caused this registration statement to be signed
on its behalf by the undersigned, thereto duly authorized.

                              NATURAL MICROSYSTEMS CORPORATION



Dated:  January 19, 1999      By: /s/ Robert P. Schechter
                                  -----------------------
                              Name:  Robert P. Schechter
                              Title: President and Chief
                                     Executive Officer

                                       5
<PAGE>
 
                                 EXHIBIT INDEX

     The following designated exhibits are filed herewith:

1.  Rights Agreement, dated as of
    December 23, 1998 (the "Rights
    Agreement"), between Natural
    MicroSystems Corporation and State
    Street Bank and Trust Company, as
    Rights Agent.

2.  Form of Certificate of Designations,
    Rights, Preferences and Privileges of
    the Series A Junior Preferred Stock,
    par value $.01 per share, of the
    Company (attached as Exhibit A to the
    Rights Agreement).

3.  Form of Rights Certificate (attached
    as Exhibit B to the Rights
    Agreement).  Pursuant to the Rights
    Agreement, printed Rights
    Certificates will not be mailed until
    the Distribution Date (as defined in
    the Rights Agreement).

4.  Summary of Rights (attached as
    Exhibit C to the Rights Agreement).

                                       6

<PAGE>
 
                       NATURAL MICROSYSTEMS CORPORATION

                                      and

                      STATE STREET BANK AND TRUST COMPANY

                                 Rights Agent



                               RIGHTS AGREEMENT

                         Dated as of December 23, 1998
<PAGE>
 
                               TABLE OF CONTENTS
 
                                                                           Page
Section 1.  Certain Definitions............................................  1
 
Section 2.  Appointment of Rights Agent....................................  5
 
Section 3.  Issuance of Rights Certificates................................  6
 
Section 4.  Form of Rights Certificates....................................  7
 
Section 5.  Countersignature and Registration..............................  8
 
Section 6.  Transfer, Split Up, Combination and Exchange of Rights 
            Certificates; Mutilated, Destroyed, Lost or Stolen 
            Rights Certificates............................................  9
 
Section 7.  Exercise of Rights; Purchase Price; Expiration Date of Rights..  9
 
Section 8.  Cancellation and Destruction of Rights Certificates............ 11
 
Section 9.  Reservation and Availability of Preferred Shares............... 12
 
Section 10. Preferred Shares Record Date................................... 13
 
Section 11. Adjustment of Purchase Price; Number of Shares or 
            Number of Rights............................................... 13
 
Section 12. Certificate of Adjusted Purchase Price or Number of Shares..... 21
 
Section 13. Consolidation, Merger or Sale or Transfer of Assets or 
            Earning Power.................................................. 22
 
Section 14. Fractional Rights and Fractional Shares........................ 25
 
Section 15. Rights of Action............................................... 25
 
Section 16. Agreement of Rights Holders.................................... 26
 
Section 17. Rights Certificate Holder Not Deemed a Stockholder............. 26
 
Section 18. Concerning the Rights Agent.................................... 26
 
Section 19. Merger or Consolidation or Change of Name of Rights Agent...... 27
 
Section 20. Duties of Rights Agent......................................... 28
 
Section 21. Change of Rights Agent......................................... 30
<PAGE>
 
Section 22. Issuance of New Rights Certificate............................. 31
 
Section 23. Redemption..................................................... 31
 
Section 24. Exchange....................................................... 32
 
Section 25. Notice of Certain Events....................................... 34
 
Section 26. Notices........................................................ 34
 
Section 27. Supplements and Amendments..................................... 35
 
Section 28. Successors..................................................... 35
 
Section 29. Determinations and Actions by the Board of Directors, etc...... 35
 
Section 30. Benefits of this Agreement..................................... 36
 
Section 31. Severability................................................... 36
 
Section 32. Governing Law.................................................. 36
 
Section 33. Counterparts................................................... 36
 
Section 34. Descriptive Headings........................................... 36
 

EXHIBITS

     Exhibit A   Form of Certificate of Designations
     Exhibit B   Form of Rights Certificate
     Exhibit C   Summary of Rights
<PAGE>
 
                                RIGHTS AGREEMENT
                                ----------------


     Rights Agreement, dated as of December 23, 1998 (this "Agreement"), between
Natural MicroSystems Corporation, a Delaware corporation (the "Company"), and
State Street Bank and Trust Company, a Massachusetts trust company (the "Rights
Agent").

     On December 23, 1998 (the "Rights Dividend Declaration Date"), the Board of
Directors of the Company authorized and declared a dividend of one Preferred
Share purchase right (a "Right") for each Common Share (as hereinafter defined)
of the Company outstanding as of the Close of Business (as hereinafter defined)
on January 4, 1999 (the "Record Date"), each right representing the right to
purchase one one-thousandth of a share of Series A Junior Preferred Stock (as
such number may be adjusted pursuant to the provisions of this Agreement),
having the rights, preferences and privileges set forth in the form of
Certificate of Designations, Rights, Preferences and Privileges of Series A
Junior Preferred Stock attached hereto as Exhibit A, upon the terms and subject
to the conditions herein set forth, and further authorized and directed the
issuance of one Right (as such number may be adjusted pursuant to the provisions
of this Agreement) with respect to each Common Share that shall become
outstanding between the Record Date and the earlier of the Distribution Date and
the Expiration Date (as such terms are hereinafter defined), and in certain
circumstances after the Distribution Date.

     NOW, THEREFORE, in consideration of the promises and the mutual agreements
herein set forth, the parties hereby agree as follows:

      Section 1.    Certain Definitions.  For purposes of this Agreement, the
                    -------------------                                      
following terms have the meanings indicated:

          (a) "Acquiring Person" shall mean any Person who or which, together
with all Affiliates and Associates of such Person, shall be the Beneficial Owner
of 15% or more of the Common Shares then outstanding, but shall not include the
Company, any Subsidiary of the Company or any employee benefit plan of the
Company or of any Subsidiary of the Company, or an entity holding Common Shares
for or pursuant to the terms of any such plan.  Notwithstanding the foregoing,
no Person shall be deemed to be an Acquiring Person either (i) as the result of
an acquisition of Common Shares by the Company which, by reducing the number of
shares outstanding, increases the proportionate number of shares beneficially
owned by such Person to 15% or more of the Common Shares of the Company then
outstanding;  provided, however, that if a Person shall become the Beneficial
              --------  -------                                              
Owner of 15% of more of the Common Shares of the Company then outstanding by
reason of share purchases by the Company and shall, after such share purchases
by the Company, become the Beneficial Owner of any additional Common Shares of
the Company, then such Person shall be deemed to be an Acquiring Person, or (ii)
if within eight days after such Person would otherwise become an Acquiring
Person (but for the operation of this clause (ii)), such Person notifies the
Board of Directors that such Person did so inadvertently and within two days
after such notification, such Person is the Beneficial Owner of less than 15% of
the outstanding Common Shares.
<PAGE>
 
          (b) "Affiliate" and "Associate" shall have the respective meanings
ascribed to such terms in Rule 12b-2 of the General Rules and Regulations under
the Securities Exchange Act of 1934, as amended (the "Exchange Act"), as in
effect on the date of this Agreement.

          (c) A Person shall be deemed the "Beneficial Owner" of and shall be
deemed to "beneficially own" any securities:

              (i) which such Person or any of such Person's Affiliates or 
Associates beneficially owns, directly or indirectly, for purposes of Section
13(d) of the Exchange Act and Rule 13d-3 thereunder (or any comparable or
successor law or regulation);

              (ii) which such Person or any of such Person's Affiliates or
Associates has (A) the right to acquire (whether such right is exercisable
immediately or only after the passage of time) pursuant to any agreement,
arrangement or understanding (other than customary agreements with and between
underwriters and selling group members with respect to a bona fide public
offering of securities), or upon the exercise of conversion rights, exchange
rights, rights (other than the Rights), warrants or options, or otherwise;
provided, however, that a Person shall not be deemed pursuant to this Section
- --------  -------                                                            
1(c)(ii)(A) the Beneficial Owner of, or to beneficially own, (1) securities
tendered pursuant to a tender or exchange offer made by or on behalf of such
Person or any of such Person's Affiliates or Associates until such tendered
securities are accepted for purchase or exchange or (2) securities which a
Person or any of such Person's Affiliates or Associates may be deemed to have
the right to acquire pursuant to any merger or other acquisition agreement
between the Company and such Person (or one or more of its Affiliates or
Associates) if such agreement has been approved by the Board of Directors of the
Company prior to there being an Acquiring Person; or (B) the right to vote
pursuant to any agreement, arrangement or understanding; provided, however, that
                                                         --------  -------      
a Person shall not be deemed the Beneficial Owner of, or to beneficially own,
any security under this Section 1(c)(ii)(B) if the agreement, arrangement or
understanding to vote such security (1) arises solely from a revocable proxy or
consent given to such Person in response to a public proxy or consent
solicitation made pursuant to, and in accordance with, the applicable rules and
regulations of the Exchange Act and (2) is not also then reportable on Schedule
13D under the Exchange Act (or any comparable or successor report);  or

              (iii) which are beneficially owned, directly or indirectly, by any
other Person (or any Affiliate or Associate thereof) with which such Person or
any of such Person's Affiliates or Associates has any agreement, arrangement or
understanding (whether or not in writing) (other than customary agreements with
and between underwriters and selling group members with respect to a bona fide
public offering of securities) for the purpose of acquiring, holding, voting
(except to the extent contemplated by the proviso to Section 1(c)(ii)(B)) or
disposing of any securities of the Company; provided, however, that in no case
                                            --------  -------                 
shall an officer or director of the Company be deemed (x) the Beneficial Owner
of any securities beneficially owned by another officer or director of the
Company solely by 

                                       2
<PAGE>
 
reason of actions undertaken by such persons in their capacity as officers or
directors of the Company or (y) the Beneficial Owner of securities held of
record by the trustee of any employee benefit plan of the Company or any
Subsidiary of the Company for the benefit of any employee of the Company or any
Subsidiary of the Company, other than the officer or director, by reason of any
influence that such officer or director may have over the voting of the
securities held in the plan.

          (d) "Business Day" shall mean any day other than a Saturday, Sunday or
a day on which banking institutions in Massachusetts are authorized or obligated
by law or executive order to close.

          (e) "Close of Business" on any given date shall mean 5:00 P.M.,
Massachusetts time, on such date;  provided, however, that if such date is not a
                                   --------  -------                            
Business Day it shall mean 5:00 P.M., Massachusetts time, on the next succeeding
Business Day.

          (f) "Common Shares" when used with reference to the Company shall mean
the shares of Common Stock of the Company, $0.01 par value.  "Common Shares"
when used with reference to any Person other than the Company shall mean the
capital stock (or equity interest) with the greatest voting power of such other
Person or, if such other Person is a Subsidiary of another Person, the Person or
Persons which ultimately control such first-mentioned Person.

          (g) "Continuing Director" shall mean (i) any member of the Board of
directors of the Company, while a member of the Board, who is not an Acquiring
Person, or an Affiliate or Associate of an Acquiring Person, or a representative
of an Acquiring Person or of any such Affiliate or Associate, and who was a
member of the Board prior to the date of this Agreement, or (ii) any Person who
subsequently becomes a member of the Board, while a member of the Board, who is
not an Acquiring Person, or an Affiliate or Associate of an Acquiring Person, or
a representative of an Acquiring Person or of any such Affiliate or Associate,
if such Person's nomination for election or election to the Board is recommended
or approved by a majority of the Continuing Directors.

          (h) "Distribution Date" shall mean the earlier of (i) the Close of
Business on the tenth day (or such later date as may be determined by action of
a majority of Continuing Directors then in office) after the Shares Acquisition
Date (or, if the tenth day after the Shares Acquisition Date occurs before the
Record Date, the Close of Business on the Record Date) or (ii) the Close of
Business on the tenth day (or such later date as may be determined by action of
a majority of Continuing Directors then in office) after the date that a tender
or exchange offer by any Person (other than the Company, any Subsidiary of the
Company, any employee benefit plan of the Company or of any Subsidiary of the
Company, or any Person or entity organized, appointed or established by the
Company for or pursuant to the terms of any such plan) is first published or
sent or given within the meaning of Rule 14d-2(a) of the General Rules and
Regulations under the Exchange Act, if, assuming the successful consummation
thereof, such person would be the Beneficial Owner of 30% or more of the shares
of Common Stock then outstanding.

                                       3
<PAGE>
 
          (i) "Equivalent Shares" shall mean Preferred Shares and any other
class or series of capital stock of the Company which is entitled to participate
in dividends and other distributions, including distributions upon the
liquidation, dissolution or winding up of the Company, on a proportional basis
with the Common Shares.  In calculating the number of any class or series of
Equivalent Shares for purposes of Section 11 of this Agreement, the number of
shares, or fractions of a share, of such class or series of capital stock that
is entitled to the same dividend or distribution as a whole Common Share shall
be deemed to be one share.

          (j) "Expiration Date" shall mean the earliest of (i) the Close of
Business on the Final Expiration Date, (ii) the Redemption Date, (iii) the time
at which the Board of Directors orders the exchange of the Rights as provided in
Section 24 hereof or (iv) the consummation of a transaction contemplated by
Section 13(d) hereof.

          (k) "Final Expiration Date" shall mean December 23, 2008.

          (l) "Permitted Offer" shall mean a tender offer for all outstanding
Common Shares made in the manner prescribed by Section 14(d) of the Exchange Act
and the rules and regulations promulgated thereunder;  provided, however, that
                                                       --------  -------      
such tender offer occurs at a time when Continuing Directors are in office and a
majority of the Continuing Directors then in office has determined that the
offer is both adequate and otherwise in the best interests of the Company and
its stockholders (taking into account all factors that such Continuing Directors
deem relevant, including without limitation prices that could reasonably be
achieved if the Company or its assets were sold on an orderly basis designed to
realize maximum value).

          (m) "Person" shall mean any individual, firm, corporation or other
entity, and shall include any successor (by merger or otherwise) of such entity.

          (n) "Preferred Shares" shall mean shares of Series A Junior Preferred
Stock of the Company.

          (o) "Purchase Price" shall have the meaning set forth in Section 4(a)
hereof.

          (p) "Record Date" shall have the meaning set forth in the recitals at
the beginning of this Agreement.

          (q) "Redemption Date" shall mean the time at which the Board of
Directors of the Company orders redemption of the Rights as provided in Section
23 hereof.

                                       4
<PAGE>
 
          (r) "Redemption Price" shall have the meaning set forth in Section
23(a) hereof.

          (s) "Rights Dividend Declaration Date" shall have the meaning set
forth in the recitals at the beginning of this Agreement.

          (t) "Section 13 Event" shall mean any event described in clause (i),
(ii) or (iii) of Section 13(a) hereof.

          (u) "Shares Acquisition Date" shall mean the first date of public
announcement (which, for purposes of this definition, shall include, without
limitation, a report filed pursuant to Section 13(d) under the Exchange Act) by
the Company or an Acquiring Person that an Acquiring Person has become such;
that, if such person is determined not to have become an Acquiring
- -------- ----                                                              
Person pursuant to Section 1(a)(ii) hereof, then no Shares Acquisition Date
shall be deemed to have occurred.

          (v) "Subsidiary" of any Person shall mean any corporation or other
entity of which an amount of voting securities sufficient to elect a majority of
the directors or Persons having similar authority of such corporation or other
entity is beneficially owned, directly or indirectly, by such Person, or any
corporation or other entity otherwise controlled by such Person.

          (w) "Total Exercise Price" shall have the meaning set forth in Section
4(a) hereof.

          (x) "Trading Day" shall have the meaning set forth in Section 11(d)
hereof.

          (y) A "Triggering Event" shall be deemed to have occurred upon any
Person (other than the Company, any Subsidiary of the Company, any employee
benefit plan of the Company or any Subsidiary of the Company, or any entity
holding Common Shares for or pursuant to the terms of any such plan), together
with all Affiliates and Associates of such Person, becoming an Acquiring Person.

      Section 2.    Appointment of Rights Agent.  The Company hereby appoints
                    ---------------------------                              
the Rights Agent to act as agent for the Company and the holders of the Rights
(who, in accordance with Section 3 hereof, shall, prior to the Distribution
Date, also be the holders of the Common Shares) in accordance with the terms and
conditions hereof, and the Rights Agent hereby accepts such appointment.  The
Company may from time to time appoint such co-Rights Agents as it may deem
necessary or desirable, upon ten (10) days' prior written notice to the Rights
Agent.  The Rights Agent shall have no duty to supervise, and shall in no event
be liable for, the acts or omissions of any such co-Rights Agent.

                                       5
<PAGE>
 
      Section 3.    Issuance of Rights Certificates.
                    ------------------------------- 

          (a) Until the Distribution Date, (i) the Rights will be evidenced
(subject to the provisions of Section 3(b) and 3(c) hereof) by the certificates
for Common Shares registered in the names of the holders thereof (which
certificates shall also be deemed to be Rights Certificates) and not by separate
Rights Certificates and (ii) the right to receive Rights Certificates will be
transferable only in connection with the transfer of Common Shares. Until the
earlier of the Distribution Date or the Expiration Date, the surrender for
transfer of such certificates for Common Shares shall also constitute the
surrender for transfer of the Rights associated with the Common Shares
represented thereby.  As soon as practicable after the Distribution Date, the
Company will prepare and execute, the Rights Agent will countersign, and the
Company will send or cause to be sent (and the Rights Agent will, if requested,
send) by first-class, postage-prepaid mail, to each record holder of Common
Shares as of the close of business on the Distribution Date, at the address of
such holder shown on the records of the Company, a Rights Certificate in
substantially the form of Exhibit B hereto (a "Rights Certificate"), evidencing
one Right for each Common Share so held, subject to adjustment as provided
herein.  In the event that an adjustment in the number of Rights per Common
Share has been made pursuant to Section 11(a)(i), Section 11(i) or Section 11(p)
hereof, then at the time of distribution of the Rights Certificates, the Company
shall make the necessary and appropriate rounding adjustments (in accordance
with Section 14(a) hereof) so that Rights Certificates representing only whole
numbers of Rights are distributed and cash is paid in lieu of any fractional
Rights.  As of the Distribution Date, the Rights will be evidenced solely by
such Rights Certificates and may be transferred by the transfer of the Rights
Certificates as permitted hereby, separated and apart from any transfer of one
or more Common Shares, and the holders of such Rights Certificates as listed in
the records of the Company or any transfer agent or registrar for the Rights
shall be the record holders thereof.

          (b) On the Record Date or as soon as practicable thereafter, the
Company will send a copy of a Summary of Rights in substantially the form of
Exhibit C hereto (the "Summary of Rights"), by first-class, postage-prepaid
mail, to each record holder of Common Shares as of the close of business on the
Record Date, at the address of such holder shown on the records of the Company.

          (c) Unless the Board of Directors, by resolution adopted at or before
the time of the issuance (including pursuant to the exercise of rights under the
Company's benefit plans) of any Common Shares, specifies to the contrary, Rights
shall be issued in respect of all Common Shares that are issued after the Record
Date but prior to the earlier of the Distribution Date or the Expiration Date
or, in certain circumstances provided in Section 22 hereof, after the
Distribution Date.  Certificates representing such Common Shares shall also be
deemed to be certificates for Rights, and shall bear the following legend:

     This certificate also evidences and entitles the holder hereof to certain
     rights as set forth in a Rights Agreement between Natural MicroSystems
     Corporation and State

                                       6
<PAGE>
 
     Street Bank and Trust Company as the Rights Agent, dated as of December 23,
     1998 (the "Rights Agreement"), the terms of which are hereby incorporated
     herein by reference and a copy of which is on file at the principal
     executive offices of Natural MicroSystems Corporation. Under certain
     circumstances, as set forth in the Rights Agreement, such Rights will be
     evidenced by separate certificates and will no longer be evidenced by this
     certificate. Natural MicroSystems Corporation will mail to the holder of
     this certificate a copy of the Rights Agreement without charge after
     receipt of a written request therefor. Under certain circumstances set
     forth in the Rights Agreement, Rights issued to, or held by, any Person who
     is, was or becomes an Acquiring Person or any Affiliate or Associate
     thereof (as such terms are defined in the Rights Agreement), whether
     currently held by or on behalf of such Person or by any subsequent holder,
     may become null and void.

With respect to such certificates containing the foregoing legend, until the
earlier of (i) the Distribution Date or (ii) the Expiration Date, the Rights
associated with the Common Shares represented by such certificates shall be
evidenced by such certificates alone, and the surrender for transfer of any such
certificate shall also constitute the transfer of the Rights associated with the
Common Shares represented thereby.  In the event that the Company purchases or
acquires any Common Shares after the Record Date but prior to the Distribution
Date, any Rights associated with such Common Shares shall be deemed cancelled
and retired so that the Company shall not be entitled to exercise any Rights
associated with the Common Shares which are no longer outstanding.

      Section 4.    Form of Rights Certificates.
                    --------------------------- 

          (a) The Rights Certificates (and the forms of election to purchase
Common Shares and of assignment to be printed on the reverse thereof) shall be
substantially in the form of Exhibit B hereto and may have such marks of
identification or designation and such legends, summaries or endorsements
printed thereon as the Company may deem appropriate and as are not inconsistent
with the provisions of this Agreement, or as may be required to comply with any
applicable law or with any rule or regulation made pursuant thereto or with any
rule or regulation of any stock exchange on which the Rights may from time to
time be listed, or to conform to usage.  Subject to the provisions of Section 11
and Section 22 hereof, the Rights Certificates, whenever distributed, shall be
dated as of the Record Date (or in the case of Rights issued with respect to
Common Shares issued by the Company after the Record Date, as of the date of
issuance of such Common Shares) and on their face shall entitle the holders
thereof to purchase such number of one-thousandths of a Preferred Share as shall
be set forth therein at the price set forth therein (such exercise price per one
one-thousandth of a Preferred Share being hereinafter referred to as the
"Purchase Price" and the aggregate exercise price of all Preferred Shares
issuable upon exercise of one Right being hereinafter referred to as the "Total
Exercise Price"), but the number and type of securities purchasable upon the
exercise of each Right and the Purchase Price shall be subject to adjustment as
provided herein.

                                       7
<PAGE>
 
          (b) Any Rights Certificate issued pursuant to Section 3(a) or Section
22 hereof that represents Rights beneficially owned by:  (i) an Acquiring Person
or any Associate or Affiliate of an Acquiring Person, (ii) a transferee of an
Acquiring Person (or of any such Associate or Affiliate) who becomes a
transferee after the Acquiring Person becomes such or (iii) a transferee of an
Acquiring Person (or of any such Associate or Affiliate) who becomes a
transferee prior to or concurrently with the Acquiring Person becoming such and
receives such Rights pursuant to either (A) a transfer (whether or not for
consideration) from the Acquiring Person to holders of equity interests in such
Acquiring Person or to any Person with whom such Acquiring Person has any
continuing agreement, arrangement or understanding regarding the transferred
Rights or (B) a transfer which the Board of Directors of the Company has
determined is part of a plan, arrangement or understanding which has as a
primary purpose or effect avoidance of Section 7(e) hereof, and any Rights
Certificate issued pursuant to Section 6 or Section 11 hereof upon transfer,
exchange, replacement or adjustment of any other Rights Certificate referred to
in this sentence, shall contain (to the extent feasible) the following legend:

     The Rights represented by this Rights Certificate are or were beneficially
     owned by a Person who was or became an Acquiring Person or an Affiliate or
     Associate of an Acquiring Person (as such terms are defined in the Rights
     Agreement).  Accordingly, this Rights Certificate and the Rights
     represented hereby may become null and void in the circumstances specified
     in Section 7(e) of the Rights Agreement.

      Section 5.    Countersignature and Registration.
                    --------------------------------- 

          (a) The Rights Certificates shall be executed on behalf of the Company
by its Chairman of the Board, its Chief Executive Officer, its President or any
Vice President, either manually or by facsimile signature, and by the Secretary
or an Assistant Secretary of the Company, either manually or by facsimile
signature, and shall have affixed thereto the Company's seal (if any) or a
facsimile thereof.  The Rights Certificates shall be manually countersigned by
the Rights Agent and shall not be valid for any purpose unless countersigned.
In case any officer of the Company who shall have signed any of the Rights
Certificates shall cease to be such officer of the Company before
countersignature by the Rights Agent and issuance and delivery by the Company,
such Rights Certificates nevertheless may be countersigned by the Rights Agent
and issued and delivered by the Company with the same force and effect as
through the person who signed such Rights Certificates had not ceased to be such
officer of the Company;  and any Rights Certificate may be signed on behalf of
the Company by any person who, at the actual date of the execution of such
Rights Certificate, shall be a proper officer of the Company to sign such Rights
Certificate, although at the date of the execution of this Rights Agreement any
such person was not such an officer.

          (b) Following the Distribution Date, the Rights Agent will keep or
cause to be kept, at its office designated for such purposes, books for
registration and transfer of the Rights Certificates issued hereunder.  Such
books shall show the names and addresses of the 

                                       8
<PAGE>
 
respective holders of the Rights Certificates, the number of Rights evidenced on
its face by each of the Rights Certificates and the date of each of the Rights
Certificates.

      Section 6.    Transfer, Split Up, Combination and Exchange of Rights
                    ------------------------------------------------------
Certificates; Mutilated, Destroyed, Lost or Stolen Rights Certificates.
- ---------------------------------------------------------------------- 

          (a) Subject to the provisions of Sections 7(e), 14 and 24 hereof, at
any time after the Close of Business on the Distribution Date, and at or prior
to the Close of Business on the Expiration Date, any Rights Certificate or
Rights Certificates may be transferred, split up, combined or exchanged for
another Rights Certificate or Rights Certificates, entitling the registered
holder to purchase a like number of one-thousandths of a Preferred Share (or,
following a Triggering Event, other securities, cash or other assets, as the
case may be) as the Rights Certificate or Rights Certificates surrendered then
entitled such holder to purchase.  Any registered holder desiring to transfer,
split up, combine or exchange any Rights Certificate or Rights Certificates
shall make such request in writing delivered to the Rights Agent, and shall
surrender the Rights Certificate or Rights Certificates to be transferred, split
up, combined or exchanged at the office of the Rights Agent designated for such
purpose.  Neither the Rights Agent nor the Company shall be obligated to take
any action whatsoever with respect to the transfer of any such surrendered
Rights Certificate until the registered holder shall have completed and signed
the certificate contained in the form of assignment on the reverse side of such
Rights Certificate and shall have provided such additional evidence of the
identity of the Beneficial Owner (or former Beneficial Owner) or Affiliates or
Associates thereof as the Company shall reasonably request.  Thereupon the
Rights Agent shall, subject to Sections 7(e), 14 and 24 hereof, countersign and
deliver to the Person entitled thereto a Rights Certificate or Rights
Certificates, as the case may be, as so requested.  The Company may require
payment of a sum sufficient to cover any tax or governmental charge that may be
imposed in connection with any transfer, split up, combination or exchange of
Rights Certificates.

          (b) Upon receipt by the Company and the Rights Agent of evidence
reasonably satisfactory to them of the loss, theft, destruction or mutilation of
a Rights Certificate, and, in case of loss, theft or destruction, of indemnity
or security reasonably satisfactory to them, and, at the Company's request,
reimbursement to the Company and the Rights Agent and cancellation of the Rights
Certificate if mutilated, the Company will make and deliver a new Rights
Certificate of like tenor to the Rights Agent for delivery to the registered
holder in lieu of the Rights Certificate so lost, stolen, destroyed or
mutilated.

      Section 7.    Exercise of Rights; Purchase Price; Expiration Date of
                    ------------------------------------------------------
Rights.
- ------ 

          (a) Subject to Section 7(e) hereof, the registered holder of any
Rights Certificate may exercise the Rights evidenced thereby (except as
otherwise provided herein) in whole or in part at any time after the
Distribution Date upon surrender of the Rights Certificate, with the form of
election to purchase on the reverse side thereof duly executed, to the Rights
Agent at the office of the Rights Agent designated for such purpose, together
with payment of the Purchase Price for each one-thousandth of a Preferred Share
as to which the Rights are exercised, at or prior to the Expiration Date.

                                       9
<PAGE>
 
          (b) The Purchase Price for each one-thousandth of a Preferred Share
issuable pursuant to the exercise of a Right shall initially be $60, shall be
subject to adjustment from time to time as provided in Sections 11 and 13 hereof
and shall be payable in lawful money of the Untied States of America in
accordance with paragraph (c) below.

          (c) Upon receipt of a Rights Certificate representing exercisable
Rights, with the form of election to purchase duly executed, accompanied by
payment of the Purchase Price for the number of one-thousandths of a Preferred
Share (or other securities or property, as the case may be) to be purchased and
an amount equal to any applicable transfer tax required to be paid by the holder
of such Rights Certificate in accordance with Section 9 hereof in cash, or by
certified check or cashier's check payable to the order of the Company, the
Rights Agent shall, subject to Section 20(k) hereof, thereupon promptly (i) (A)
requisition from any transfer agent of the Preferred Shares (or make available,
if the Rights Agent is the transfer agent for the Preferred Shares) a
certificate or certificates for the number of one-thousandths of a Preferred
Share to be purchased and the Company hereby irrevocably authorizes its transfer
agent to comply with all such requests or (B) if the Company shall have elected
to deposit the total number of one-thousandths of a Preferred Share issuable
upon exercise of the Rights hereunder with a depositary agent, requisition from
the depositary agent of depositary receipts representing such number of one-
thousandths of a Preferred Share as are to be purchased (in which case
certificates for the Preferred Shares represented by such receipts shall be
deposited by the transfer agent with the depositary agent) and the Company
hereby directs the depositary agent to comply with such request, (ii) when
appropriate, requisition from the Company the amount of cash to be paid in lieu
of issuance of fractional shares in accordance with Section 14 hereof, (iii)
after receipt of such certificates or depositary receipts, cause the same to be
delivered to or upon the order of the registered holder of such Rights
Certificate, registered in such name or names as may be designated by such
holder and (iv) when appropriate, after receipt thereof, deliver such cash to or
upon the order of the registered holder of such Rights Certificate.  The payment
of the Purchase Price (as such amount may be reduced (including to zero)
pursuant to Section 11(a)(iv) hereof) may be made in cash or by certified bank
check or bank draft payable to the order of the Company.  In the event that the
Company is obligated to issue other securities of the Company, pay cash and/or
distribute other property pursuant to Section 11(a) hereof, the Company will
make all arrangements necessary so that such other securities, cash and/or other
property are available for distribution by the Rights Agent, if and when
appropriate.

          (d) In case the registered holder of any Rights Certificate shall
exercise less than all the Rights evidenced thereby, a new Rights Certificate
evidencing Rights equivalent to the Rights remaining unexercised shall be issued
by the Rights Agent to the registered holder of such Rights Certificate or to
his or her duly authorized assigns, subject to the provisions of Section 14
hereof.

                                       10
<PAGE>
 
          (e) Notwithstanding anything in this Agreement to the contrary, from
and after the first occurrence of a Triggering Event or a Section 13 Event, any
Rights beneficially owned by (i) an Acquiring Person or an Associate or
Affiliate of an Acquiring Person, (ii) a transferee of an Acquiring Person (or
of any such Associate or Affiliate) who becomes a transferee after the Acquiring
Person becomes such (a "Post Transferee"), (iii) a transferee of an Acquiring
Person (or of any such Associate or Affiliate) who becomes a transferee prior to
or concurrently with the Acquiring Person becoming such and receives such Rights
pursuant to either (A) a transfer (whether or not for consideration) from the
Acquiring Person to holders of equity interests in such Acquiring Person or to
any Person with whom the Acquiring Person has any continuing agreement,
arrangement or understanding regarding the transferred Rights or (B) a transfer
which the Board of Directors of the Company has determined is part of a plan,
arrangement or understanding which has as a primary purpose or effect the
avoidance of this Section 7(e) (a "Prior Transferee") or (iv) any subsequent
transferee receiving transferred Rights from a Post Transferee or a Prior
Transferee, either directly or through one or more intermediate transferees,
shall become null and void without any further action and no holder of such
Rights shall have any rights whatsoever with respect to such Rights, whether
under any provision of this Agreement or otherwise.  The Company shall use all
reasonable efforts to ensure that the provisions of this Section 7(e) and
Section 4(b) hereof are complied with, but shall have no liability to any holder
of Rights Certificates or to any other Person as a result of its failure to make
any determinations with respect to an Acquiring Person or any of such Acquiring
Person's Affiliates, Associates or transferees hereunder.

          (f) Notwithstanding anything in this Agreement to the contrary,
neither the Rights Agent nor the Company shall be obligated to undertake any
action with respect to a registered holder upon the occurrence of any purported
exercise as set forth in this Section 7 unless such registered holder shall have
(i) completed and signed the certificate contained in the form of election to
purchase set forth on the reverse side of the Rights Certificate surrendered for
such exercise and (ii) provided such additional evidence of the identity of the
Beneficial Owner (or former Beneficial Owner) or Affiliates or Associates
thereof as the Company shall reasonably request.

      Section 8.    Cancellation and Destruction of Rights Certificates.  All
                    ---------------------------------------------------      
Rights Certificates surrendered for the purpose of exercise, transfer, split up,
combination or exchange shall, if surrendered to the Company or to any of its
agents, be delivered to the Rights Agent for cancellation or in cancelled form,
or, if surrendered to the Rights Agent, shall be cancelled by it, and no Rights
Certificates shall be issued in lieu thereof except as expressly permitted by
any of the provisions of this Agreement.  The Company shall deliver to the
Rights Agent for cancellation and retirement, and the Rights Agent shall so
cancel and retire, any other Rights Certificate purchased or acquired by the
Company otherwise than upon the exercise thereof.  The Rights Agent shall
deliver all cancelled Rights Certificates to the Company, or shall, at the
written request of the Company, destroy such cancelled Rights Certificates, and
in such case shall deliver a certificate of destruction thereof to the Company.

                                       11
<PAGE>
 
      Section 9.    Reservation and Availability of Preferred Shares.
                    ------------------------------------------------ 

          (a) The Company covenants and agrees that it will use its best efforts
to cause to be reserved and kept available out of and to the extent of its
authorized and unissued shares of preferred stock not reserved for another
purpose (and, following the occurrence of a Triggering Event, out of its
authorized and unissued shares of Common Shares and/or other securities), the
number of Preferred Shares (and, following the occurrence of the Triggering
Event, Common Shares and/or other securities) that will be sufficient to permit
the exercise in full of all outstanding Rights.

          (b) If the Company shall hereafter list any of its Preferred Shares on
a national securities exchange, then so long as the Preferred Shares (and,
following the occurrence of a Triggering Event, Common Shares and/or other
securities) issuable and deliverable upon exercise of the Rights may be listed
on a national securities exchange, the Company shall use its best efforts to
cause, from and after such time as the Rights become exercisable (but only to
the extent that it is reasonably likely that the Rights will be exercised), all
shares reserved for such issuance to be listed on such exchange upon official
notice of issuance upon such exercise.

          (c) The Company shall use its best efforts to (i) file, as soon as
practicable following the earliest date after the first occurrence of a
Triggering Event in which the consideration to be delivered by the Company upon
exercise of the Rights has been determined in accordance with Section 11(a)(iv)
hereof, or as soon as is required by law following the Distribution Date, as the
case may be, a registration statement under the Securities Act of 1933, as
amended (the "Securities Act"), with respect to the securities purchasable upon
exercise of the Rights on an appropriate form, (ii) cause such registration
statement to become effective as soon as practicable after such filing, and
(iii) cause such registration statement to remain effective (with a prospectus
at all times meeting the requirements of the Securities Act) until the earlier
of (A) the date as of which the Rights are no longer exercisable for such
securities and (B) the date of expiration of the Rights.  The Company may
temporarily suspend, for a period not to exceed ninety (90) days after the date
set forth in clause (i) of the first sentence of this Section 9(c), the
exercisability of the Rights in order to prepare and file such registration
statement and permit it to become effective. Upon any such suspension, the
Company shall issue a public announcement stating, and notify the Rights Agent,
that the exercisability of the Rights has been temporarily suspended, as well as
a public announcement and notification to the Rights Agent at such time as the
suspension is no longer in effect.  The Company will also take such action as
may be appropriate under, or to ensure compliance with, the securities or "blue
sky" laws of the various states in connection with the exercisability of the
Rights.  Notwithstanding any provision of this Agreement to the contrary, the
Rights shall not be exercisable in any jurisdiction, unless the requisite
qualification in such jurisdiction shall have been obtained, or an exemption
therefrom shall be available, and until a registration statement has been
declared effective.

                                       12
<PAGE>
 
          (d) The Company covenants and agrees that it will take all such action
as may be necessary to ensure that all Preferred Shares delivered upon exercise
of Rights shall, at the time of delivery of the certificates for such Preferred
Shares (subject to payment of the Purchase Price), be duly and validly
authorized and issued and fully paid and nonassessable shares.

          (e) The Company further covenants and agrees that it will pay when due
and payable any and all federal and state transfer taxes and charges which may
be payable in respect of the original issuance or delivery of the Rights
Certificates or of any Preferred Shares upon the exercise of Rights.  The
Company shall not, however, be required to pay any transfer tax which may be
payable in respect of any transfer or delivery of Rights Certificates to a
person other than, or the issuance or delivery of certificates or depositary
receipts for the Preferred Shares in a name other than that of, the registered
holder of the Rights Certificate evidencing Rights surrendered for exercise or
to issue or to deliver any certificates or depositary receipts for Preferred
Shares upon the exercise of any Rights until any such tax shall have been paid
(any such tax being payable by the holder of such Rights Certificate at the time
of surrender) or until it has been established to the Company's satisfaction
that no such tax is due.

      Section 10.   Preferred Shares Record Date.  Each Person in whose name any
                    ----------------------------                                
certificate for a number of one-thousandths of a Preferred Share is issued upon
the exercise of Rights shall for all purposes be deemed to have become the
holder of record of Preferred Shares represented thereby on, and such
certificate shall be dated, the date upon which the Rights Certificate
evidencing such Rights was duly surrendered and payment of the Purchase Price
multiplied by the number of one-thousandths of a Preferred Share with respect to
which the Rights have been exercised (and any applicable transfer taxes) was
made; provided, however, that if the date of such surrender and payment is a
      --------  -------                                                     
date upon which the Preferred Shares transfer books of the Company are closed,
such person shall be deemed to have become the record holder of such shares on,
and such certificate shall be dated, the next succeeding Business Day on which
the Preferred Shares transfer books of the Company are open.  Prior to the
exercise of the Rights evidenced thereby, the holder of a Rights Certificate
shall not be entitled to any rights of a holder of Preferred Shares for which
the Rights shall be exercisable, including, without limitation, the right to
vote, to receive dividends or other distributions or to exercise any preemptive
rights, and shall not be entitled to receive any notice of any proceedings of
the Company, except as provided herein.

      Section 11.   Adjustment of Purchase Price; Number of Shares or Number of
                    -----------------------------------------------------------
Rights. The Purchase Price, the number and kind of shares or other property
- ------                                                                     
covered by each Right and the number of Rights outstanding are subject to
adjustment from time to time as provided in this Section 11.

          (a)  (i)  In the event the Company shall at any time after the date of
this Agreement (A) declare a dividend on the Common Shares payable in Common
Shares, (B) subdivide the outstanding Common Shares, (C) combine the outstanding
Common Shares (by 

                                       13
<PAGE>
 
reverse stock split or otherwise) into a smaller number of Common Shares, or (D)
issue any shares of its capital stock in a reclassification of the Common Shares
(including any such reclassification in connection with a consolidation or
merger in which the Company is the continuing or surviving corporation), then,
in each such event, except as otherwise provided in this Section 11(a) and
Section 7(e) hereof: (1) each of the Rights outstanding at the time of the
record date for such dividend or the effective date of such subdivision,
combination or reclassification shall be proportionately adjusted to that number
of Rights (calculated to the nearest one ten-thousandth (1/10,000) of a Right)
equal to a fraction (the "Exchange Ratio"), the numerator of which shall be the
total number of Common Shares or shares of capital stock issued in such
reclassification of the Common Shares outstanding immediately following such
time and the denominator of which shall be the total number of Common Shares
outstanding immediately prior to such time, and the number of Rights that shall
thereafter be issued with respect to each Common Share or share of such other
capital stock that shall become outstanding thereafter prior to the Distribution
Date shall be equal to the total number of outstanding Rights immediately after
such event (as adjusted pursuant to this clause (1)) divided by the total number
of outstanding Common Shares or shares of such other capital stock immediately
after such event (subject to further adjustment pursuant to the provisions of
this Agreement); (2) the Purchase Price in effect at the time of the record date
for such dividend or of the effective date of such subdivision, combination or
reclassification shall be adjusted so that the Purchase Price thereafter shall
equal the result obtained by dividing the Purchase Price in effect immediately
prior to such time by the Exchange Ratio; provided, however, that in no event
                                          --------  -------
shall the consideration to be paid upon the exercise of one Right be less than
the aggregate par value of the shares of capital stock of the Company issuable
upon exercise of such Right; and (3) the number of Common Shares or shares of
such other capital stock issuable upon the exercise of each Right shall remain
unchanged immediately after such event, but, in the event of a reclassification,
the kind of shares issuable upon the exercise of each Right immediately after
such reclassification shall be adjusted to be the kind of shares of such other
capital stock issued in such reclassification, rather than Common Shares. If an
event occurs which would require an adjustment under both this Section 11(a)(i)
and Section 11(a)(ii) hereof, the adjustment provided for in this Section
11(a)(i) shall be in addition to, and shall be made prior to, any adjustment
required pursuant to Section 11(a)(ii) hereof.

               (ii) Subject to Section 24 of this Agreement, in the event a
Triggering Event shall have occurred, then promptly following such Triggering
Event, proper provision shall be made so that each holder of a Right, except as
provided in Section 7(e) hereof, shall thereafter have the right to receive for
each Right, upon exercise thereof in accordance with the terms of this Agreement
and payment of the then-current Total Exercise Price, in lieu of a number of
one-thousandths of a Preferred Share, such number of Common Shares of the
Company as shall equal the result obtained by multiplying the then-current
Purchase Price by the then number of one-thousandths of a Preferred Share for
which a Right was exercisable (or would have been exercisable if the
Distribution Date had occurred) immediately prior to the first occurrence of a
Triggering Event, and dividing that product by 50% of the current per share
market price (determined pursuant to Section 11(d) hereof) for Common Shares on
the date of occurrence of the Triggering Event (such number of shares being
hereinafter referred to as the "Adjustment Shares").

                                       14
<PAGE>
 
               (iii) The right to buy Common Shares of the Company pursuant to
Section 11(a)(ii) hereof shall not arise as a result of any Person becoming an
Acquiring Person through an acquisition of Common Shares pursuant to a Permitted
Offer.

               (iv) In lieu of issuing Common Shares in accordance with Section
11(a)(ii) hereof, the Company may, if the Board of Directors determines that
such action is necessary or appropriate and not contrary to the interest of
holders of Rights (and, in the event that the number of Common Shares which are
authorized by the Company's Certificate of Incorporation but not outstanding or
reserved for issuance for purposes other than upon exercise of the Rights are
not sufficient to permit the exercise in full of the Rights, or if any necessary
regulatory approval for such issuance has not been obtained by the Company, the
Company shall):  (A) determine the excess of (1) the value of the Common Shares
issuable upon the exercise of a Right (the "Current Value") over (2) the
Purchase Price (such excess, the "Spread") and (B) with respect to each Right
make adequate provision to substitute for such Common Shares, upon exercise of
the Rights (1) cash, (2) a reduction in the Purchase Price, (3) other equity
securities of the Company (including, without limitation, shares or units of
shares of any series of preferred stock which the Board of Directors of the
Company has deemed to have the same value as Common Shares (such shares or units
of shares of preferred stock are herein called "common stock equivalents"),
except to the extent that the Company has not obtained any necessary stockholder
or regulatory approval for such issuance, (4) debt securities of the Company
except to the extent that the Company has not obtained any necessary stockholder
or regulatory approval for such issuance, (5) other assets or (6) any
combination of the foregoing, having an aggregate value equal to the Current
Value, where such aggregate value has been determined by the Board of Directors
of the Company based upon the advice of a nationally recognized investment
banking firm selected by the Board of Directors of the Company; provided,
                                                                -------- 
however, if the Company shall not have made adequate provision to deliver value
- -------                                                                        
pursuant to clause (B) above within thirty (30) days following the later of (x)
the first occurrence of a Triggering Event and (y) the date on which the
Company's right of redemption pursuant to Section 23(a) expires (the later of
(x) and (y) being referred to herein as the "Section 11(a)(ii) Trigger Date"),
then the Company shall be obligated to deliver, upon the surrender for exercise
of a Right and without requiring payment of the Purchase Price, Common Shares
(to the extent available), except to the extent that the Company has not
obtained any necessary stockholder or regulatory approval for such issuance, and
then, if necessary, cash, which shares and/or cash have an aggregate value equal
to the Spread.  If the Board of Directors of the Company shall determine in good
faith that it is likely that sufficient additional Common Shares could be
authorized for issuance upon exercise in full of the Rights or that any
necessary regulatory approval for such issuance will be obtained, the thirty
(30) day period set forth above may be extended to the extent necessary, but not
more than ninety (90) days after the Section 11(a)(ii) Trigger Date, in order
that the Company may seek stockholder approval for the authorization of such
additional shares or take action to obtain such regulatory approval (such
period, as it may be

                                       15
<PAGE>
 
extended, the "Substitution Period"). To the extent that the Company determines
that some action need be taken pursuant to the first and/or second sentences of
this Section 11(a)(iv), the Company (x) shall provide, subject to Section 7(e)
hereof, that such action shall apply uniformly to all outstanding Rights and (y)
may suspend the exercisability of the Rights until the expiration of the
Substitution Period in order to seek any authorization of additional shares, to
take any action to obtain any required regulatory approval and/or to decide the
appropriate form of distribution to be made pursuant to such first sentence and
to determine the value thereof. In the event of any such suspension, the Company
shall issue a public announcement stating that the exercisability of the Rights
has been temporarily suspended, as well as a public announcement at such time as
the suspension is no longer in effect. For purposes of this Section 11(a)(iv),
the value of the Common Shares shall be the current per share market price (as
determined pursuant to Section 11(d) hereof) of the Common Shares on the Section
11(a)(ii) Trigger Date and the value of any "common stock equivalent" shall be
deemed to have the same value as the Common Shares on such date.

          (b) In case the Company shall, at any time after the date of this
Agreement, fix a record date for the issuance of rights, options or warrants to
all holders of Common Shares or of any class or series of Equivalent Shares
entitling such holders (for a period expiring within forty-five (45) calendar
days after such record date) to subscribe for or purchase Common Shares or
Equivalent Shares or securities convertible into Common Shares or Equivalent
Shares at a price per share (or having a conversion price per share, if a
security convertible into Common Shares or Equivalent Shares) less than the then
current per share market price of the Common Shares or Equivalent Shares (as
defined in Section 11(d)) on such record date, then, in each such case, the
Purchase Price to be in effect after such record date shall be determined by
multiplying the Purchase Price in effect immediately prior to such record date
by a fraction, the numerator of which shall be the number of Common Shares and
Equivalent Shares (if any) outstanding on such record date, plus the number of
Common Shares or Equivalent Shares, as the case may be, which the aggregate
offering price of the total number of Common Shares or Equivalent Shares, as the
case may be, so to be offered (and/or the aggregate initial conversion price of
the convertible securities so to be offered) would purchase at such current
market price, and the denominator of which shall be the number of Common Shares
and Equivalent Shares (if any) outstanding on such record date, plus the number
of additional Common Shares or Equivalent Shares, as the case may be, to be
offered for subscription or purchase (or into which the convertible securities
so to be offered are initially convertible).  In case such subscription price
may be paid in a consideration part or all of which shall be in a form other
than cash, the value of such consideration shall be as determined in good faith
by the Board of Directors of the Company, whose determination shall be described
in a statement filed with the Rights Agent and shall be binding on the Rights
Agent and the holders of the Rights.  Common Shares and Equivalent Shares owned
by or held for the account of the Company shall not be deemed outstanding for
the purpose of any such computation.  Such adjustment shall be made successively
whenever such a record date is fixed, and in the event that such rights, options
or warrants are not so issued, the Purchase Price shall be adjusted to be the
Purchase Price which would then be in effect if such record date had not been
fixed.

                                       16
<PAGE>
 
          (c) In case the Company shall, at any time after the date of this
Agreement, fix a record date for the making of a distribution to all holders of
the Common Shares or of any class or series of Equivalent Shares (including any
such distribution made in connection with a consolidation or merger in which the
Company is the continuing or surviving corporation) or evidences of indebtedness
or assets (other than a regular quarterly cash dividend, if any, or a dividend
payable in Common Shares) or subscription rights, options or warrants (excluding
those referred to in Section 11(b)), then, in each such case, the Purchase Price
to be in effect after such record date shall be determined by multiplying the
Purchase Price in effect immediately prior to such record date by a fraction,
the numerator of which shall be the current market price (as determined pursuant
to Section 11(d) hereof) of a Common Share or an Equivalent Share on such record
date, less the fair market value (as determined in good faith by the Board of
Directors of the Company, whose determination shall be described in a statement
filed with the Rights Agent) of the portion of the cash, assets or evidences of
indebtedness so to be distributed or of such subscription rights or warrants
applicable to a Common Share or Equivalent Share, as the case may be, and the
denominator of which shall be such current market price (as determined pursuant
to Section 11(d) hereof) of a Common Share or Equivalent Share on such record
date.  Such adjustments shall be made successively whenever such a record date
is fixed, and in the event that such distribution is not so made, the Purchase
Price shall be adjusted to be the Purchase Price which would have been in effect
if such record date had not been fixed.

          (d) For the purpose of any computation hereunder, other than
computations made pursuant to Section 11(a)(iv) hereof, the "current per share
market price" of any security (a "Security" for the purpose of this Section
11(d)) on any date shall be deemed to be the average of the daily closing prices
per share of such Security for the thirty (30) consecutive Trading Days (as such
term is hereinafter defined) immediately prior to such date, and for purposes of
computations made pursuant to Section 11(a)(iv) hereof, the "current per share
market price" of any Security on any date shall be deemed to be the average of
the daily closing prices per share of such Security for the ten (10) consecutive
Trading Days immediately prior to such date; provided, however, that in the
                                             --------  -------             
event that the current per share market price of the Security is determined
during a period following the announcement by the issuer of such Security of (i)
a dividend or distribution on such Security payable in shares of such Security
or securities convertible into such shares or (ii) any subdivision, combination
or reclassification of such Security, and prior to the expiration of the
requisite thirty (30) Trading Days or ten (10) Trading Day period, after the ex-
dividend date for such dividend or distribution, or the record date for such
subdivision, combination or reclassification, then, and in each such case, the
current per share market price shall be appropriately adjusted to reflect the
current market price per share equivalent of such Security.  The closing price
for each day shall be the last sale price, regular way, or, in case no such sale
takes place on such day, the average of the closing bid and asked prices,
regular way, in either case as reported in the principal consolidated
transaction reporting system with respect to securities listed or admitted to
trading on the New York Stock Exchange or, if the Security is not listed or
admitted to trading on the New York Stock Exchange, as reported in the principal
consolidated transaction reporting system with respect to securities listed on
the

                                       17
<PAGE>
 
principal national securities exchange on which the Security is listed or
admitted to trading or, if the Security is not listed or admitted to trading on
any national securities exchange, the last sale price or, if such last sale
price is not reported, the average of the high bid and low asked prices in the
over-the-counter market, as reported by the National Association of Securities
Dealers, Inc. Automated Quotations System ("NASDAQ") or such other system then
in use, or, if on any such date the Security is not quoted by any such
organization, the average of the closing bid and asked prices as furnished by a
professional market maker making a market in the Security selected by the Board
of Directors of the Company.  If on any such date no market maker is making a
market in the Common Shares, the fair value of such shares on such date as
determined in good faith by the Board of Directors of the Company shall be used.
The term "Trading Day" shall mean a day on which the principal national
securities exchange on which the Security is listed or admitted to trading is
open for the transaction of business or, if the Security is not listed or
admitted to trading on any national securities exchange, a Business Day.  If the
Common Shares are not publicly held or so listed or traded, "current per share
market price" shall mean the fair value per share as determined in good faith by
the Board of Directors of the Company, whose determination shall be described in
a statement filed with the Rights Agent and shall be conclusive for all
purposes.

          (e) Anything herein to the contrary notwithstanding, no adjustment in
the Purchase Price shall be required unless such adjustment would require an
increase or decrease of at least 1% in the Purchase Price; provided, however,
                                                           --------  ------- 
that any adjustments which by reason of this Section 11(e) are not required to
be made shall be carried forward and taken into account in any subsequent
adjustment.  All calculations under this Section 11 shall be made to the nearest
cent or to the nearest ten-thousandth of a Common Share or other share or one
hundred-thousandth of a Preferred Share, as the case may be. Notwithstanding the
first sentence of this Section 11(e), any adjustment required by this Section 11
shall be made no later than the earlier of (i) three (3) years from the date of
the transaction which requires such adjustment or (ii) the Expiration Date.

          (f) If as a result of an adjustment made pursuant to Section 11(a) or
13(a) hereof, the holder of any Right thereafter exercised shall become entitled
to receive any shares of capital stock other than Preferred Shares, thereafter
the number of such other shares so receivable upon exercise of any Right and if
required, the Purchase Price thereof, shall be subject to adjustment from time
to time in a manner and on terms as nearly equivalent as practicable to the
provisions with respect to the Common Shares contained in sections 11(a), (b),
(c), (e), (h), (i), (j), (k), (l) and (m), and the provisions of Sections 7, 9,
10, 13 and 14 with respect to the Preferred Shares shall apply on like terms to
any such other shares.

          (g) All Rights originally issued by the Company subsequent to any
adjustment made to the Purchase Price hereunder shall evidence the right to
purchase, at the adjusted Purchase Price, the number of one-thousandths of a
Preferred Share purchasable from time to time hereunder upon exercise of the
Rights, all subject to further adjustment as provided herein.

                                       18
<PAGE>
 
          (h) Unless the Company shall have exercised its election as provided
in Section 11(i), upon each adjustment of the Purchase Price as a result of the
calculations made in Section 11(b), each Right outstanding immediately prior to
the making of such adjustment shall thereafter evidence the right to purchase,
at the adjusted Purchase Price, that number of Preferred Shares (calculated to
the nearest one hundred-thousandth of a share) obtained by (i) multiplying (x)
the number of Preferred Shares covered by a Right immediately prior to this
adjustment, by (y) the Purchase Price in effect immediately prior to such
adjustment of the Purchase Price, and (ii) dividing the product so obtained by
the Purchase Price in effect immediately after such adjustment of the Purchase
Price.

          (i) The Company may elect on or after the date of any adjustment of
the Purchase Price as a result of the calculations made in Section 11(b) to
adjust the number of Rights, in substitution for any adjustment in the number of
Preferred Shares purchasable upon the exercise of a Right.  Each of the Rights
outstanding after such adjustment of the number of Rights shall be exercisable
for the number of one-thousandths of a Preferred Share for which a Right was
exercisable immediately prior to such adjustment.  Each Right held of record
prior to such adjustment of the number of Rights shall become that number of
Rights (calculated to the nearest ten-thousandth) obtained by dividing the
Purchase Price in effect immediately prior to adjustment of the Purchase Price
by the Purchase Price in effect immediately after adjustment of the Purchase
Price.  The Company shall make a public announcement of its election to adjust
the number of Rights, indicating the record date for the adjustment, and, if
known at the time, the amount of the adjustment to be made.  This record date
may be the date on which the Purchase Price is adjusted or any day thereafter,
but, if the Rights Certificates have been issued, shall be at least ten (10)
days later than the date of the public announcement.  If Rights Certificates
have been issued, upon each adjustment of the number of Rights pursuant to this
Section 11(i), the Company shall, as promptly as practicable, cause to be
distributed to holders of record of Rights Certificates on such record date
Rights Certificates evidencing, subject to Section 14 hereof, the additional
Rights to which such holders shall be entitled as a result of such adjustment,
or, at the option of the Company, shall cause to be distributed to such holders
of record in substitution and replacement for the Rights Certificates held by
such holders prior to the date of adjustment, and upon surrender thereof, if
required by the Company, new Rights Certificates evidencing all the Rights to
which such holders shall be entitled after such adjustment.  Rights Certificates
so to be distributed shall be issued, executed and countersigned in the manner
provided for herein (and may bear, at the option of the Company, the adjusted
Purchase Price) and shall be registered in the names of the holders of record of
Rights Certificates on the record date specified in the public announcement.

          (j) Irrespective of any adjustment or change in the Purchase Price or
the number of Preferred Shares issuable upon the exercise of the Rights, the
Rights Certificates theretofore and thereafter issued may continue to express
the Purchase Price per one one-thousandth of a Preferred Share and the number of
one-thousandths of a Preferred Share which were expressed in the initial Rights
Certificates issued hereunder.

                                       19
<PAGE>
 
          (k) Before taking any action that would cause an adjustment reducing
the Purchase Price below the par or stated value, if any, of the number of one-
thousandths of a Preferred Share issuable upon exercise of the Rights, the
Company shall take any corporate action which may, in the opinion of its
counsel, be necessary in order that the Company may validly and legally issue as
fully paid and nonassessable shares such number of one-thousandths of a
Preferred Share at such adjusted Purchase Price.

          (l) In any case in which this Section 11 shall require that an
adjustment in the Purchase Price be made effective as of a record date for a
specified event, the Company may elect to defer until the occurrence of such
event the issuing to the holder of any Right exercised after such record date of
the number of one-thousandths of a Preferred Share and other capital stock or
securities of the Company, if any, issuable upon such exercise over and above
the number of one-thousandths of a Preferred Share and other capital stock or
securities of the Company, if any, issuable upon such exercise on the basis of
the Purchase Price in effect prior to such adjustment; provided, however, that
                                                       --------  -------      
the Company shall deliver to such holder a due bill or other appropriate
instrument evidencing such holder's right to receive such additional shares
(fractional or otherwise) upon the occurrence of the event requiring such
adjustment.

          (m) Anything in this Section 11 to the contrary notwithstanding, prior
to the Distribution Date, the Company shall be entitled to make such reductions
in the Purchase Price, in addition to those adjustments expressly required by
this Section 11, as and to the extent that it in its sole discretion shall
determine to be advisable in order that any (i) consolidation or subdivision of
the Preferred or Common Shares, (ii) issuance wholly for cash of any Preferred
or Common Shares at less than the current market price, (iii) issuance wholly
for cash of Preferred or Common Shares or securities which by their terms are
convertible into or exchangeable for Preferred or Common Shares, (iv) stock
dividends or (v) issuance of rights, options or warrants referred to in this
Section 11, hereafter made by the Company to holders of its Preferred or Common
Shares shall not be taxable to such stockholders.

          (n) The Company covenants and agrees that it shall not, at any time
after the Distribution Date, effect or permit to occur any Triggering Event or
Section 13 Event, if (i) at the time or immediately after such Triggering Event
or Section 13 Event there are any rights, warrants or other instruments or
securities outstanding or agreements in effect which would substantially
diminish or otherwise eliminate the benefits intended to be afforded by the
Rights or (ii) prior to, simultaneously with or immediately after such Section
13 Event, the stockholders of the Person who constitutes, or would constitute,
the "Principal Party" for purposes of Section 13(b) hereof shall have received a
distribution of Rights previously owned by such Person or any of its Affiliates
and Associates.

                                       20
<PAGE>
 
          (o) The Company covenants and agrees that, after the Distribution
Date, it will not, except as permitted by Sections 23, 24 or 27 hereof, take (or
permit to be taken) any action if at the time such action is taken it is
reasonably foreseeable that such action will diminish substantially or otherwise
eliminate the benefits intended to be afforded by the Rights.

          (p) Anything in this Agreement to the contrary notwithstanding, in the
event the Company shall at any time after the date of this Agreement (A) declare
a dividend on the Preferred Shares payable in Preferred Shares, (B) subdivide
the outstanding Preferred Shares, (C) combine the outstanding Preferred Shares
(by reverse stock split or otherwise) into a smaller number of Preferred Shares,
or (D) issue any shares of its capital stock in a reclassification of the
Preferred Shares (including any such reclassification in connection with a
consolidation or merger in which the Company is the continuing or surviving
corporation), then, in each such event, except as otherwise provided in this
Section 11 and Section 7(e) hereof:  (1) each of the Rights outstanding at the
time of the record date for such dividend or the effective date of such
subdivision, combination or reclassification shall be proportionately adjusted
to that number of Rights (calculated to the nearest one ten-thousandth
(1/10,000) of a Right) equal to a fraction (the "Exchange Fraction"), the
numerator of which shall be the total number of Preferred Shares or shares of
capital stock issued in such reclassification of the Preferred Shares
outstanding immediately following such time and the denominator of which shall
be the total number of Preferred Shares outstanding immediately prior to such
time, and the number of Rights that shall thereafter be issued with respect to
each Common Share or share of other capital stock that shall be issued in a
reclassification of the Common Shares prior to the Distribution Date shall be
equal to the total number of outstanding Rights immediately after such event (as
adjusted pursuant to this clause(1)) divided by the total number of outstanding
Common Shares or shares of such other capital stock immediately after such event
(subject to further adjustment pursuant to the provisions of this Agreement);
(2) the Purchase Price in effect at the time of the record date for such
dividend or of the effective date of such subdivision, combination or
reclassification shall be adjusted so that the Purchase Price thereafter shall
equal the result obtained by dividing the Purchase Price in effect immediately
prior to such time by the Exchange Fraction; provided, however, that in no event
                                             --------  -------                  
shall the consideration to be paid upon the exercise of one Right be less than
the aggregate par value of the shares of capital stock of the Company issuable
upon exercise of such Right; and (3) the number of one-thousandths of a
Preferred Share or share of such other capital stock issuable upon the exercise
of each Right shall remain unchanged immediately after such event, but, in the
event of a reclassification, the kind of shares issuable upon the exercise of
each Right immediately after such reclassification shall be adjusted to be the
kind of shares of such other capital stock issued in such reclassification,
rather than Preferred Shares.

      Section 12.   Certificate of Adjusted Purchase Price or Number of Shares.
                    ---------------------------------------------------------- 
Whenever an adjustment is made as provided in Sections 11 and 13 hereof, the
Company shall promptly (a) prepare a certificate setting forth such adjustment
and a brief statement of the facts accounting for such adjustment, (b) file with
the Rights Agent and with each transfer agent for the Preferred Shares a copy of
such certificate and (c) mail a brief 

                                       21
<PAGE>
 
summary thereof to each holder of a Rights Certificate in accordance with
Section 26 hereof. Notwithstanding the foregoing sentence, the failure of the
Company to make such certification or give such notice shall not affect the
validity of such adjustment or the force or effect of the requirement for such
adjustment. The Rights Agent shall be fully protected in relying on any such
certificate and on any adjustment contained therein and shall not be deemed to
have knowledge of such adjustment unless and until it shall have received such
certificate.

      Section 13.   Consolidation, Merger or Sale or Transfer of Assets or
                    ------------------------------------------------------
Earning Power.
- ------------- 

          (a) In the event that, following the Shares Acquisition Date, directly
or indirectly:

               (i) the Company shall consolidate with, or merge with and into,
any other Person (other than a Subsidiary of the Company in a transaction the
principal purpose of which is to change the state of incorporation of the
Company or which complies with Section 11(o) hereof);

               (ii) any Person (other than a Subsidiary of the Company in a
transaction that complies with Section 11(o) hereof) shall consolidate with the
Company, or merge with and into the Company and the Company shall be the
continuing or surviving corporation of such consolidation or merger; or

               (iii) the Company shall sell or otherwise transfer (or one or 
more of its Subsidiaries shall sell or otherwise transfer), in one or more
transactions, assets or earning power aggregating 50% or more of the assets or
earning power of the Company and its Subsidiaries (taken as a whole) to any
other Person or Persons (other than the Company or one or more of its wholly
owned Subsidiaries in one or more transactions, each of which complies with
Section 11(o) hereof);

               then, and in each such case, proper provision shall be made so
that

          (A) each holder of a Right (except as otherwise provided herein) shall
thereafter have the right to receive, upon the exercise thereof in accordance
with the terms of this Agreement, such number of validly authorized and issued,
fully paid and nonassessable Common Shares of the Principal Party (as
hereinafter defined), free of any liens, encumbrances, rights of first refusal
or other adverse claims, as shall be equal to the result obtained by (1)
multiplying the then current Purchase Price by the number of one-thousandths of
a Preferred Share for which a Right was exercisable immediately prior to the
first occurrence of a Section 13 Event (or, if a Triggering Event has occurred
prior to the first occurrence of a Section 13 Event, multiplying the number of
such one-thousandths of a Preferred Share for which a Right was exercisable
immediately prior to the first occurrence of a Triggering Event by the Purchase
Price in effect immediately prior to such first occurrence) and (2) dividing
that product (which, following the first occurrence of a 

                                       22
<PAGE>
 
Section 13 Event, shall be referred to as the "Total Exercise Price" for each
Right and for all purposes of this Agreement) by 50% of the current per share
market price (determined pursuant to Section 11(d) hereof) of the Common Shares
of such Principal Party on the date of consummation of such Section 13 Event;

               (B) such Principal Party shall thereafter be liable for, and 
shall assume, by virtue of such Section 13 Event, all the obligations and duties
of the Company pursuant to this Agreement;

               (C) the term "Company" shall thereafter be deemed to refer to 
such Principal Party, it being specifically intended that the provisions of
Section 11 hereof shall apply only to such Principal Party following the first
occurrence of a Section 13 Event;

               (D) such Principal Party shall take such steps (including, but 
not limited to, the reservation of a sufficient number of its Common Shares) in
connection with the consummation of any such transaction as may be necessary to
assure that the provisions hereof shall thereafter be applicable, as nearly as
reasonably may be, in relation to its Common Shares thereafter deliverable upon
the exercise of the Rights.

          (b) "Principal Party" shall mean, in the case of any transaction
described in clause (i), (ii) or (iii) of Section 13(a), the Person or Acquiring
Person referred to therein (or such Person's or Acquiring Person's successor,
including, if applicable, the Company, if it is the surviving corporation),
provided, however, that in any such case, (i) if such Person is a direct or
- --------  -------                                                          
indirect Subsidiary of another Person, "Principal Party" shall refer to such
other Person and (ii) in case such Person is a Subsidiary, directly or
indirectly, of more than one Person, "Principal Party" shall refer to whichever
of such Persons is the issuer of the Common Shares having the greatest aggregate
value, and provided, further, that for purposes of transactions described in
           --------  -------                                                
clause (iii) hereof, "Principal Party" shall refer to that Person receiving the
greatest portion of the assets or earning power transferred pursuant to such
transaction or transactions.

          (c) If, for any reason, the Rights cannot be exercised for Common
Shares of such Principal Party as provided in Section 13(a), then each holder of
Rights shall have the right to exchange its Rights for cash from such Principal
Party in an amount equal to the number of Common Shares that it would otherwise
be entitled to purchase times 50% of the current per share market price, as
determined pursuant to Section 11(d) hereof, of such Common Shares of such
Principal Party.  If, for any reason, the foregoing formulation cannot be
applied to determine the cash amount into which the Rights are exchangeable,
then the Board of Directors, based upon the advice of one or more nationally
recognized investment banking firms, and based upon the total value of the
Company, shall determine such amount reasonably and with good faith to the
holders of Rights.  Any such determination shall be final and binding on the
Rights Agent.

                                       23
<PAGE>
 
          (d) Notwithstanding anything in this Agreement to the contrary,
Section 13 shall not be applicable to a transaction described in clauses (i) and
(ii) of Section 13(a) if:  (i) such transaction is consummated with a Person or
Persons who acquired Common Shares pursuant to a Permitted Offer (or a wholly-
owned Subsidiary of any such Person or Persons); (ii) the price per share of
Common Shares offered in such transaction is not less than the price per share
of Common Shares paid to all holders of Common Shares whose shares were
purchased pursuant to such Permitted Offer; and (iii) the form of consideration
being offered to the remaining holders of Common Shares pursuant to such
transaction is the same form as the form of consideration paid pursuant to such
Permitted Offer.  Upon consummation of any such transaction contemplated by this
Section 13(d), all rights hereunder shall expire.

          (e) The Company shall not consummate any Section 13 Event unless the
Principal Party shall have a sufficient number of authorized Common Shares that
have not been issued or reserved for issuance to permit the exercise in full of
the Rights in accordance with this Section 13 and unless prior thereto the
Company and such issuer shall have executed and delivered to the Rights Agent a
supplemental agreement confirming that such Principal Party shall, upon
consummation of such Section 13 Event, assume this Agreement in accordance with
Sections 13(a) and (b) hereof, that all rights of first refusal or preemptive
rights in respect of the issuance of Common Shares of such Principal Party upon
exercise of outstanding Rights have been waived, that there are no rights,
warrants, instruments or securities outstanding or any agreements or
arrangements which, as a result of the consummation of such transaction, would
eliminate or substantially diminish the benefits intended to be afforded by the
Rights and that such transaction shall not result in a default by such Principal
Party under this Agreement, and further providing that, as soon as practicable
after the date of such Section 13 Event, such Principal Party will:

               (i) prepare and file a registration statement under the 
Securities Act with respect to the Rights and the securities purchasable upon
exercise of the Rights on an appropriate form, use its best efforts to cause
such registration statement to become effective as soon as practicable after
such filing and use its best efforts to cause such registration statement to
remain effective (with a prospectus at all times meeting the requirements of the
Securities Act) until the Expiration Date, and similarly comply with applicable
state securities laws;

               (ii) use its best efforts to list (or continue the listing of)
the Rights and the securities purchasable upon exercise of the Rights on a
national securities exchange or to meet the eligibility requirements for
quotation on NASDAQ; and

               (iii)  deliver to holders of the Rights historical financial
statements for such Principal Party which comply in all respects with the
requirements for registration on Form 10 (or any successor form) under the
Exchange Act.

          In the event that at any time after the occurrence of a Triggering
Event some or all of the Rights shall not have been exercised at the time of a
transaction described in this Section 13, the Rights which have not theretofore
been exercised shall thereafter be exercisable in the manner described in
Section 13(a) (without taking into account any prior adjustment required by
Section 11(a)(ii)).

                                       24
<PAGE>
 
          (f) The provisions of this Section 13 shall similarly apply to
successive mergers or consolidations or sales or other transfers.

      Section 14.   Fractional Rights and Fractional Shares.
                    --------------------------------------- 

          (a) The Company shall not be required to issue fractions of Rights or
to distribute Rights Certificates which evidence fractional Rights.  In lieu of
such fractional Rights, there shall be paid to the registered holders of the
Rights Certificates with regard to which such fractional Rights would otherwise
be issuable, an amount in cash equal to the same fraction of the current market
value of a whole Right.  For the purposes of this Section 14(a), the current
market value of a whole Right shall be the closing price of the Rights for the
Trading Day immediately prior to the date on which such fractional Rights would
have been otherwise issuable, as determined pursuant to the second sentence of
Section 11(d) hereof.

          (b) The Company shall not be required to issue fractions of Preferred
Shares (other than fractions that are integral multiples of one one-thousandth
of a Preferred Share) upon exercise of the Rights or to distribute certificates
which evidence fractional Preferred Shares (other than fractions that are
integral multiples of one one-thousandth of a Preferred Share).  In lieu of
fractional Preferred Shares that are not integral multiples of one one-
thousandth of a Preferred Share, the Company shall pay to the registered holders
of Rights Certificates at the time such Rights are exercised as herein provided
an amount in cash equal to the same fraction of the current market value of a
Common Share.  For purposes of this Section 14(b), the current market value of a
Common Share shall be the closing price of a Common Share (as determined
pursuant to the second sentence of Section 11(d) hereof) for the Trading Day
immediately prior to the date of such exercise.

          (c) The holder of a Right by the acceptance of the Right expressly
waives his or her right to receive any fractional Rights or any fractional
shares upon exercise of a Right.

      Section 15.   Rights of Action.  All rights of action in respect of this
                    ----------------                                          
Agreement, excepting the rights of action given to the Rights Agent under
Section 18 hereof, are vested in the respective registered holders of the Rights
Certificates (and, prior to the Distribution Date, the registered holders of the
Common Shares); and any registered holder of any Rights Certificate (or, prior
to the Distribution Date, of the Common Shares), without the consent of the
Rights Agent or of the holder of any other Rights Certificate (or, prior to the
Distribution Date, of the Common Shares), may, in his or her own behalf and for
his or her own benefit, enforce, and may institute and maintain any suit, action
or proceeding against the Company to enforce, or otherwise act in respect of,
his or her right to exercise the Rights evidenced by such Rights Certificate in
the manner provided in such Rights Certificate

                                       25
<PAGE>
 
and in this Agreement. Without limiting the foregoing or any remedies available
to the holders of Rights, it is specifically acknowledged that the holders of
Rights would not have an adequate remedy at law for any breach of this Agreement
and will be entitled to specific performance of the obligations under and
injunctive relief against actual or threatened violators of, the obligations of
any Person subject to this Agreement.

      Section 16.   Agreement of Rights Holders.  Every holder of a Right, by
                    ---------------------------                              
accepting the same, consents and agrees with the Company and the Rights Agent
and with every other holder of a Right that:

          (a) prior to the Distribution Date, the Rights will be transferable
only in connection with the transfer of the Common Shares;

          (b) after the Distribution Date, the Rights Certificates are
transferable only on the registry books of the Rights Agent if surrendered at
the principal office or offices of the Rights Agent designated for such purposes
duly endorsed or accompanied by a proper instrument of transfer and with the
appropriate forms and certificates fully executed; and

          (c) subject to Sections 6(a) and 7(f) hereof, the Company and the
Rights Agent may deem and treat the person in whose name the Rights Certificate
(or, prior to the Distribution Date, the associated Common Shares certificate)
is registered as the absolute owner thereof and of the Rights evidenced thereby
(notwithstanding any notations of ownership or writing on the Rights
Certificates or the associated Common Shares certificate made by anyone other
than the Company or the Rights Agent) for all purposes whatsoever, and neither
the Company nor the Rights Agent shall be affected by any notice to the
contrary.

      Section 17.   Rights Certificate Holder Not Deemed a Stockholder.  No
                    --------------------------------------------------     
holder, as such, of any Rights Certificate shall be entitled to vote, receive
dividends or be deemed for any purpose the holder of the Preferred Shares or any
other securities of the Company which may at any time be issuable on the
exercise of the Rights represented thereby, nor shall anything contained herein
or in any Rights Certificate be construed to confer upon the holder of any
Rights Certificate, as such, any of the rights of a stockholder of the Company
or any right to vote for the election of directors or upon any matter submitted
to stockholders at any meeting thereof, or to give or withhold consent to any
corporate action, or to receive notice of meetings or other actions affecting
stockholders (except as provided in Section 25 hereof), or to receive dividends
or subscription rights, or otherwise, until the Right or Rights evidenced by
such Rights Certificate shall have been exercised in accordance with the
provisions hereof.

      Section 18.   Concerning the Rights Agent.
                    --------------------------- 

          (a) The Company agrees to pay to the Rights Agent reasonable
compensation for all services rendered by it hereunder and, from time to time,
on demand

                                       26
<PAGE>
 
of the Rights Agent, its reasonable expenses and counsel fees and other
disbursements incurred in the administration and execution of this Agreement and
the exercise and performance of its duties hereunder. The Company also agrees to
indemnify the Rights Agent for, and to hold it harmless against any loss,
liability or expense, incurred without gross negligence, bad faith or willful
misconduct on the part of the Rights Agent, for anything done or omitted by the
Rights Agent in connection with the acceptance and administration of this
Agreement, including the costs and expenses of defending against any claim of
liability in the premises.

          (b) The Rights Agent shall be protected and shall incur no liability
for, or in respect of any action taken, suffered or omitted by it in connection
with, its administration of this Agreement in reliance upon any Rights
Certificate or certificate for the Common Shares or for other securities of the
Company, instrument of assignment or transfer, power of attorney, endorsement,
affidavit, letter, notice, direction, consent, certificate, statement or other
paper or document believed by it to be genuine and to be signed, executed and,
where necessary, verified or acknowledged, by the proper Person or Persons, or
otherwise upon the advice of counsel as set forth in Section 20 hereof.

      Section 19.   Merger or Consolidation or Change of Name of Rights Agent.
                    --------------------------------------------------------- 

          (a) Any corporation into which the Rights Agent or any successor
Rights Agent may be merged or with which it may be consolidated, or any
corporation resulting from any merger or consolidation to which the Rights Agent
or any successor Rights Agent shall be a party, or any corporation succeeding to
the corporate trust business of the Rights Agent or any successor Rights Agent,
shall be the successor to the Rights Agent under this Agreement without the
execution or filing of any paper or any further act on the part of any of the
parties hereto; provided, however, that such corporation would be eligible for
                --------  -------                                             
appointment as a successor Rights Agent under the provisions of Section 21
hereof.  In case at the time such successor Rights Agent shall succeed to the
agency created by this Agreement, any of the Rights Certificates shall have been
countersigned but not delivered, any such successor Rights Agent may adopt the
counter-signature of the predecessor Rights Agent and deliver such Rights
Certificates so countersigned; and in case at that time any of the Rights
Certificates shall not have been countersigned, any successor Rights Agent may
countersign such Rights Certificates either in the name of the predecessor
Rights Agent or in the name of the successor Rights Agent; and in all such cases
such Rights Certificates shall have the full force provided in the Rights
Certificates and in this Agreement.

          (b) In case at any time the name of the Rights Agent shall be changed
and at such time any of the Rights Certificates shall have been countersigned
but not delivered, the Rights Agent may adopt the countersignature under its
prior name and deliver Rights Certificates so countersigned; and in case at that
time any of the Rights Certificates shall not have been countersigned, the
Rights Agent may countersign such Rights Certificates either in its prior name
or in its changed name; and in all such cases such Rights Certificates shall
have the full force provided in the Rights Certificates and in this Agreement.

                                       27
<PAGE>
 
      Section 20.   Duties of Rights Agent.  The Rights Agent undertakes the
                    ----------------------                                  
duties and obligations imposed by this Agreement upon the following terms and
conditions, by all of which the Company and the holders of Rights Certificates,
by their acceptance thereof, shall be bound:

          (a) The Rights Agent may consult with legal counsel (who may be legal
counsel for the Company), and the opinion or advice of such counsel shall be
full and complete authorization and protection to the Rights Agent as to any
action taken or omitted by it in good faith and in accordance with such opinion
or advice.

          (b) Whenever in the performance of its duties under this Agreement the
Rights Agent shall deem it necessary or desirable that any fact or matter
(including, without limitation, the identity of any Acquiring Person and the
determination of "current per share market price") be proved or established by
the Company prior to taking or suffering any action hereunder, such fact or
matter (unless other evidence in respect thereof be herein specifically
prescribed) may be deemed to be conclusively proved and established by a
certificate signed by any one of the Chairman of the Board, the Chief Executive
Officer, the President, any Vice President, the Chief Financial Officer, the
Secretary or any Assistant Secretary of the Company and delivered to the Rights
Agent; and such certificate shall be full authorization to the Rights Agent for
any action taken or suffered in good faith by it under the provisions of this
Agreement in reliance upon such certificate.

          (c) The Rights Agent shall be liable hereunder to the Company and any
other Person only for its own gross negligence, bad faith or willful misconduct.

          (d) The Rights Agent shall not be liable for or by reason of any of
the statements of fact or recitals contained in this Agreement or in the Rights
Certificates (except its countersignature thereof) or be required to verify the
same, but all such statements and recitals are and shall be deemed to have been
made by the Company only.

          (e) The Rights Agent shall not be under any responsibility in respect
of the validity of this Agreement or the execution and delivery hereof (except
the due execution hereof by the Rights Agent) or in respect of the validity or
execution of any Rights Certificate (except its countersignature thereof); nor
shall it be responsible for any breach by the Company of any covenant or
condition contained in this Agreement or in any Rights Certificate; nor shall it
be responsible for any change in the exercisability of the Rights or any
adjustment in the terms of the Rights (including the manner, method or amount
thereof) provided for in Sections 3, 11, 13, 23 or 24, or the ascertaining of
the existence of facts that would require any such change or adjustment (except
with respect to the exercise of Rights evidenced by Rights Certificates after
receipt by the Rights Agent of a certificate furnished pursuant to Section 12
describing such change or adjustment); nor shall it by any act hereunder be
deemed to make any representation or warranty as to the authorization or
reservation of any Preferred Shares to be issued pursuant to this Agreement or
any Rights Certificate or as to whether any Preferred Shares will, when issued,
be validly authorized and issued, fully paid and nonassessable.

                                       28
<PAGE>
 
          (f) The Company agrees that it will perform, execute, acknowledge and
deliver or cause to be performed, executed, acknowledged and delivered all such
further and other acts, instruments and assurances as may reasonably be required
by the Rights Agent for the carrying out or performing by the Rights Agent of
the provisions of this Agreement.

          (g) The Rights Agent is hereby authorized and directed to accept
instructions with respect to the performance of its duties hereunder from any
one of the Chairman of the Board, the Chief Executive Officer, the President,
any Vice President, the Chief Financial Officer, the Secretary or any Assistant
Secretary of the Company, and to apply to such officers for advice or
instructions in connection with its duties, and it shall not be liable for any
action taken or suffered by it in good faith in accordance with instructions of
any such officer or for any delay in acting while waiting for those
instructions.  Any application by the Rights Agent for written instructions from
the Company may, at the option of the Rights Agent, set forth in writing any
action proposed to be taken or omitted by the Rights Agent under this Rights
Agreement and the date on and/or after which such action shall be taken or such
omission shall be effective.  The Rights Agent shall not be liable for any
action taken by, or omission of, the Rights Agent in accordance with a proposal
included in any such application on or after the date specified in such
application (which date shall not be less than five (5) Business Days after the
date any officer of the Company actually receives such application, unless any
such officer shall have consented in writing to an earlier date) unless, prior
to taking any such action (or the effective date in the case of an omission),
the Rights Agent shall have received written instructions in response to such
application specifying the action to be taken or omitted.

          (h) The Rights Agent and any stockholder, director, officer or
employee of the Rights Agent may buy, sell or deal in any of the Rights or other
securities of the Company or become pecuniarily interested in any transaction in
which the Company may be interested, or contract with or lend money to the
Company or otherwise act as fully and freely as though it were not Rights Agent
under this Agreement.  Nothing herein shall preclude the Rights Agent from
acting in any other capacity for the Company or for any other legal entity.

          (i) The Rights Agent may execute and exercise any of the rights or
powers hereby vested in it or perform any duty hereunder either itself or by or
through its attorneys or agents, and the Rights Agent shall not be answerable or
accountable for any act, default, neglect or misconduct of any such attorneys or
agents or for any loss to the Company resulting from any such act, default,
neglect or misconduct, provided reasonable care was exercised in the selection
and continued employment thereof.

          (j) No provisions of this Agreement shall require the Rights Agent to
expend or risk its own funds or otherwise incur any financial liability in the
performance of any of its duties hereunder or in the exercise of its rights if
there shall be reasonable grounds for believing that repayment of such funds or
adequate indemnification against such risk or liability is not reasonably
assured to it.

                                       29
<PAGE>
 
          (k) If, with respect to any Rights Certificate surrendered to the
Rights Agent for exercise or transfer, the certificate attached to the form of
assignment or form of election to purchase, as the case may be, has either not
been completed or indicates an affirmative response to clause 1 and/or 2
thereof, the Rights Agent shall not take any further action with respect to such
requested exercise or transfer without first consulting with the Company.

      Section 21.   Change of Rights Agent.  The Rights Agent or any successor
                    ----------------------                                    
Rights Agent may resign and be discharged from its duties under this Agreement
upon thirty (30) days' notice in writing mailed to the Company and to each
transfer agent of the Preferred Shares and the Common Shares by registered or
certified mail, and to the holders of the Rights Certificates by first-class
mail.  The Company may remove the Rights Agent or any successor Rights Agent
upon thirty (30) days' notice in writing, mailed to the Rights Agent or
successor Rights Agent, as the case may be, and to each transfer agent of the
Preferred Shares and the Common Shares by registered or certified mail, and to
the holders of the Rights Certificates by first-class mail.  If the Rights Agent
shall resign or be removed or shall otherwise become incapable of acting, the
Company shall appoint a successor to the Rights Agent.  If the Company shall
fail to make such appointment within a period of thirty (30) days after giving
notice of such removal or after it has been notified in writing of such
resignation or incapacity by the resigning or incapacitated Rights Agent or by
the holder of a Rights Certificate (who shall, with such notice, submit his or
her Rights Certificate for inspection by the Company), then the registered
holder of any Rights Certificate may apply to any court of competent
jurisdiction for the appointment of a new Rights Agent.  Any successor Rights
Agent, whether appointed by the Company or by such a court shall be a
corporation organized and doing business under the laws of the United States or
of any state of the United States, in good standing, which is authorized under
such laws to exercise corporate trust or stockholder services powers and is
subject to supervision or examination by federal or state authority and which
has at the time of its appointment as Rights Agent a combined capital and
surplus of at least $50,000,000.  After appointment, the successor Rights Agent
shall be vested with the same powers, rights, duties and responsibilities as if
it had been originally named as Rights Agent without further act or deed; but
the predecessor Rights Agent shall deliver and transfer to the successor Rights
Agent any property at the time held by it hereunder, and execute and deliver any
further assurance, conveyance, act or deed necessary for the purpose.  Not later
than the effective date of any such appointment, the Company shall file notice
thereof in writing with the predecessor Rights Agent and each transfer agent of
the Preferred Shares and the Common Shares, and mail a notice thereof in writing
to the registered holders of the Rights Certificates.  Failure to give any
notice provided for in this Section 21, however, or any defect therein, shall
not affect the legality or validity of the resignation or removal of the Rights
Agent or the appointment of the successor Rights Agent, as the case may be.

                                       30
<PAGE>
 
      Section 22.   Issuance of New Rights Certificate.  Notwithstanding any of
                    ----------------------------------                         
the provisions of this Agreement or of the Rights to the contrary, the Company
may, at its option, issue new Rights Certificates evidencing Rights in such form
as may be approved by its Board of Directors to reflect any adjustment or change
in the Purchase Price and the number or kind or class of shares or other
securities or property purchasable under the Rights Certificates made in
accordance with the provisions of this Agreement.  In addition, in connection
with the issuance or sale of Common Shares following the Distribution Date and
prior to the redemption or expiration of the Rights, the Company (a) shall, with
respect to Common Shares so issued or sold pursuant to the exercise of stock
options or under any employee plan or arrangement or upon the exercise,
conversion or exchange of securities hereinafter issued by the Company and (b)
may, in any other case, if deemed necessary or appropriate by the Board of
Directors of the Company, issue Rights Certificates representing the appropriate
number of Rights in connection with such issuance of sale; provided, however,
                                                           --------  ------- 
that (i) no such Rights Certificate shall be issued and this sentence shall be
null and void ab initio if, and to the extent that, such issuance or this
              ---------                                                  
sentence would create a significant risk of or result in material adverse tax
consequences to the Company or the Person to whom such Rights Certificate would
be issued or would create a significant risk of or result in such options' or
employee plans' or arrangements' failing to qualify for otherwise available
special tax treatment and (ii) no such Rights Certificate shall be issued if,
and to the extent that, appropriate adjustment shall otherwise have been made in
lieu of the issuance thereof.

      Section 23.   Redemption.
                    ---------- 

          (a) The Company may, at its option and with the approval of the Board
of Directors, at any time prior to the Close of Business on the earlier of (i)
the tenth day following the Shares Acquisition Date or such later date as may be
determined by action of a majority of Continuing Directors then in office and
publicly announced by the Company or (ii) the Final Expiration Date, redeem all
but not less than all the then outstanding Rights at a redemption price of $0.01
per Right, appropriately adjusted to reflect any stock split, stock dividend or
similar transaction occurring after the date hereof (such redemption price being
herein referred to as the "Redemption Price") and the Company may, at its
option, pay the Redemption Price either in Common Shares (based on the current
per share market price thereof (as determined pursuant to Section 11(d) hereof)
at the time of redemption) or cash; provided, however, if the Board of Directors
                                    --------  -------                           
of the Company authorizes redemption of the Rights on or after the time a Person
becomes an Acquiring Person, then there must be Continuing Directors then in
office and such authorization shall require the concurrence of a majority of
such Continuing Directors.

          (b) Immediately upon the action of the Board of Directors of the
Company ordering the redemption of the Rights, evidence of which shall have been
filed with the Rights Agent, and without any further action and without any
notice, the right to exercise the Rights will terminate and the only right
thereafter of the holders of Rights shall be to receive the Redemption Price.
Within ten (10) days after the action of the Board of Directors

                                       31
<PAGE>
 
ordering the redemption of the Rights, the Company shall give notice of such
redemption to the Rights Agent and the holders of the then outstanding Rights by
mailing such notice to all such holders at their last addresses as they appear
upon the registry books of the Rights Agent or, prior to the Distribution Date,
on the registry books of the transfer agent for the Common Shares. Any notice
which is mailed in the manner herein provided shall be deemed given, whether or
not the holder receives the notice. Each such notice of redemption will state
the method by which the payment of the Redemption Price will be made. Neither
the Company nor any of its Affiliates or Associates may redeem, acquire or
purchase for value any Rights at any time in any manner other than that
specifically set forth in this Section 23 or in Section 24 hereof, and other
than in connection with the purchase of Common Shares prior to the Distribution
Date.

      Section 24.   Exchange.
                    -------- 

          (a) Subject to applicable laws, rules and regulations, and subject to
subsection (c) below, the Company may, at its option, by majority vote of the
Board of Directors and a majority vote of the Continuing Directors, at any time
after the occurrence of a Triggering Event, exchange all or part of the then
outstanding and exercisable Rights (which shall not include Rights that have
become void pursuant to the provisions of Section 7(e) hereof) for Common Shares
at an exchange ratio of one Common Share per Right, appropriately adjusted to
reflect any stock split, stock dividend or similar transaction occurring after
the date hereof (such exchange ratio being hereinafter referred to as the "Ratio
of Exchange").  Notwithstanding the foregoing, the Board of Directors shall not
be empowered to effect such exchange at any time after any Person (other than
the Company, any Subsidiary of the Company, any employee benefit plan of the
Company or any such Subsidiary, or any entity holding Common Shares for or
pursuant to the terms of any such plan), together with all Affiliates and
Associates of such Person, becomes the Beneficial Owner of 50% or more of the
Common Shares then outstanding.

          (b) Immediately upon the action of the Board of Directors ordering the
exchange of any Rights pursuant to subsection (a) of this Section 24 and without
any further action and without any notice, the right to exercise such Rights
shall terminate and the only right thereafter of a holder of such Rights shall
be to receive that number of Common Shares equal to the number of such Rights
held by such holder multiplied by the Ratio of Exchange. The Company shall give
public notice of any such exchange; provided, however, that the failure to give,
                                    --------  -------                           
or any defect in, such notice shall not affect the validity of such exchange.
The Company shall mail a notice of any such exchange to all of the holders of
such Rights at their last addresses as they appear upon the registry books of
the Rights Agent.  Any notice which is mailed in the manner herein provided
shall be deemed given, whether or not the holder receives the notice.  Each such
notice of exchange will state the method by which the exchange of the Common
Shares for Rights will be effected and, in the event of any partial exchange,
the number of Rights which will be exchanged.  Any partial exchange shall be
effected pro rata based on the number of Rights (other than Rights which have
become void pursuant to the provisions of Section 7(e) hereof) held by each
holder of Rights.

                                       32
<PAGE>
 
          (c) In the event that there shall not be sufficient Common Shares
issued but not outstanding or authorized but unissued to permit any exchange of
Rights as contemplated in accordance with Section 24(a), the Company shall
either take such action as may be necessary to authorize additional Common
Shares for issuance upon exchange of the Rights or alternatively, at the option
of a majority of the Board of Directors, with respect to each Right (i) pay cash
in an amount equal to the Current Value (as hereinafter defined), in lieu of
issuing Common Shares in exchange therefor, or (ii) issue debt or equity
securities or a combination thereof, having a value equal to the Current Value,
in lieu of issuing Common Shares in exchange for each such Right, where the
value of such securities shall be determined by a nationally recognized
investment banking firm selected by the Board of Directors by majority vote of
the Board of Directors, or (iii) deliver any combination of cash, property,
Common Shares and/or other securities having a value equal to the Current Value
in exchange for each Right.  For purposes of this Section 24(c) only, the
Current Value shall mean the product of the current per share market price of
Common Shares (determined pursuant to Section 11(d) on the date of the
occurrence of the event described above in subparagraph 9(a)) multiplied by the
number of Common Shares for which the Right otherwise would be exchangeable if
there were sufficient shares available.  To the extent that the Company
determines that some action need be taken pursuant to clauses (i), (ii) or (iii)
of this Section 24(c), the Board of Directors may temporarily suspend the
exercisability of the Rights for a period of up to sixty (60) days following the
date on which the event described in Section 24(a) shall have occurred, in order
to seek any authorization of additional Common Shares and/or to decide the
appropriate form of distribution to be made pursuant to the above provision and
to determine the value thereof.  In the event of any such suspension, the
Company shall issue a public announcement stating that the exercisability of the
Rights has been temporarily suspended.

          (d) The Company shall not be required to issue fractions of Common
Shares or to distribute certificates which evidence fractional Common Shares.
In lieu of such fractional Common Shares, there shall be paid to the registered
holders of the Rights Certificates with regard to which such fractional Common
Shares would otherwise be issuable, an amount in cash equal to the same fraction
of the current per share market value of a whole Common Share (as determined
pursuant to the second sentence of Section 11(d) hereof).

          (e) The Company may, at its option, by majority vote of the Board of
Directors, at any time before any Person has become an Acquiring Person,
exchange all or part of the then outstanding Rights for rights of substantially
equivalent value, as determined reasonably and with good faith by the Board of
Directors, based upon the advice of one or more nationally recognized investment
banking firms.

          (f) Immediately upon the action of the Board of Directors ordering the
exchange of any Rights pursuant to subsection (e) of this Section 24 and without
any further action and without any notice, the right to exercise such Rights
shall terminate and the only right thereafter of a holder of such Rights shall
be to receive that number of rights in

                                       33
<PAGE>
 
exchange therefor as has been determined by the Board of Directors in accordance
with subsection (e) above. The Company shall give public notice of any such
exchange; provided, however, that the failure to give, or any defect in, such
          --------  -------
notice shall not affect the validity of such exchange. The Company shall mail a
notice of any such exchange to all of the holders of such Rights at their last
addresses as they appear upon the registry books of the transfer agent for the
Common Shares of the Company. Any notice which is mailed in the manner herein
provided shall be deemed given, whether or not the holder receives the notice.
Each such notice of exchange will state the method by which the exchange of the
Rights will be effected.

      Section 25.   Notice of Certain Events.
                    ------------------------ 

          (a) In case the Company shall propose to effect or permit to occur any
Section 13 Event, the Company shall give notice thereof to each holder of Rights
in accordance with Section 26 hereof at least twenty (20) days prior to
occurrence of such Section 13 Event.

          (b) In case any Triggering Event or Section 13 Event shall occur,
then, in any such case, the Company shall as soon as practicable thereafter give
to each holder of a Rights Certificate, in accordance with Section 26 hereof, a
notice of the occurrence of such event, which shall specify the event and the
consequences of the event to holders of Rights under Sections 11(a)(ii) and 13
hereof.

      Section 26.   Notices.  Notices or demands authorized by this Agreement to
                    -------                                                     
be given or made by the Rights Agent or by the holder of any Rights Certificate
to or on the Company shall be sufficiently given or made if sent by first-class
mail, postage prepaid, addressed (until another address is filed in writing with
the Rights Agent) as follows:

          Natural MicroSystems Corporation
          100 Crossing Boulevard
          Framingham, Massachusetts  01702
          Attention:  Chief Executive Officer

Subject to the provisions of Section 21 hereof, any notice or demand authorized
by this Agreement to be given or made by the Company or by the holder of any
Rights Certificate to or on the Rights Agent shall be sufficiently given or made
if sent by first-class mail, postage prepaid, addressed (until another address
is filed in writing with the Company) as follows:

          State Street Bank and Trust Company
          c/o EquiServe Limited Partnership
          150 Royall Street, Mail Stop 45-02-62
          Canton, MA 02021
          Attention:  Account Manager for Natural MicroSystems Corporation

                                       34
<PAGE>
 
Notices or demands authorized by this Agreement to be given or made by the
Company or the Rights Agent to the holder of any Rights Certificate shall be
sufficiently given or made if sent by first-class mail, postage prepaid,
addressed to such holder at the address of such holder as shown on the registry
books of the Company.

      Section 27.   Supplements and Amendments.  Prior to the Distribution Date,
                    --------------------------                                  
the Company may supplement or amend this Agreement in any respect without the
approval of any holders of Rights and the Rights Agent shall, if the Company so
directs, execute such supplement or amendment.  From and after the Distribution
Date, the Company and the Rights Agent may from time to time supplement or amend
this Agreement without the approval of any holders of Rights in order to (i)
cure any ambiguity, (ii) correct or supplement any provision contained herein
which may be defective or inconsistent with any other provisions herein, (iii)
shorten or lengthen any time period hereunder (which lengthening or shortening,
following the first occurrence of an event set forth in the proviso to Section
23(a) hereof, shall be effective only if there are Continuing Directors and
shall require the concurrence of a majority of such Continuing Directors) or
(iv) change or supplement the provisions hereunder in any manner that the
Company may deem necessary or desirable and that shall not adversely affect the
interests of the holders of Rights (other than an Acquiring Person or an
Affiliate or Associate of an Acquiring Person); provided, this Agreement may not
                                                --------                        
be supplemented or amended to lengthen, pursuant to clause (iii) of this
sentence, (A) a time period relating to when the Rights may be redeemed at such
time as the Rights are not then redeemable or (B) any other time period unless
such lengthening is for the purpose of protecting, enhancing, or clarifying the
rights of, and/or the benefits to, the holders of Rights.  Upon the delivery of
a certificate from an appropriate officer of the Company that states that the
proposed supplement or amendment is in compliance with the terms of this Section
27, the Rights Agent shall execute such supplement or amendment. Prior to the
Distribution Date, the interests of the holders of Rights shall be deemed
coincident with the interests of the holders of Common Shares.

      Section 28.   Successors.  All the covenants and provisions of this
                    ----------                                           
Agreement by or for the benefit of the Company or the Rights Agent shall bind
and inure to the benefit of their respective successors and assigns hereunder.

      Section 29.   Determinations and Actions by the Board of Directors, etc.
                    ---------------------------------------------------------  
For all purposes of this Agreement, any calculation of the number of Common
Shares outstanding at any particular time, including for purposes of determining
the particular percentage of such outstanding Common Shares of which any Person
is the Beneficial Owner, shall be made in accordance with the last sentence of
Rule 13d-3(d)(1)(i) of the General Rules and Regulations under the Exchange Act.
The Board of Directors of the Company (and, where specifically provided for
herein, the Continuing Directors) shall have the exclusive power and authority
to administer this Agreement and to exercise all rights and powers specifically
granted to the Board, or the Company (or, where specifically provided for
herein, the Continuing Directors), or as may be necessary or advisable in the
administration of this Agreement, including, without limitation, the right and
power to (i) interpret the provisions of this 

                                       35
<PAGE>
 
Agreement and (ii) make all determinations deemed necessary or advisable for the
administration of this Agreement (including a determination to redeem or not
redeem the Rights or to amend the Agreement). All such actions, calculations,
interpretations and determinations (including, for purposes of clause (y) below,
all omissions with respect to the foregoing) which are done or made by the Board
(or, where specifically provided for herein, by the Continuing Directors) in
good faith, shall (x) be final, conclusive and binding on the Company, the
Rights Agent, the holders of the Rights Certificates and all other parties and
(y) not subject the Board or the Continuing Directors to any liability to the
holders of the Rights.

      Section 30.   Benefits of this Agreement.  Nothing in this Agreement shall
                    --------------------------                                  
be construed to give to any Person other than the Company, the Rights Agent and
the registered holders of the Rights Certificates (and, prior to the
Distribution Date, the Common Shares) any legal or equitable right, remedy or
claim under this Agreement; but this Agreement shall be for the sole and
exclusive benefit of the Company, the Rights Agent and the registered holders of
the Rights Certificates (and, prior to the Distribution Date, the Common
Shares).

      Section 31.   Severability.  If any term, provision, covenant or
                    ------------                                      
restriction of this Agreement is held by a court of competent jurisdiction or
other authority to be invalid, void or unenforceable, the remainder of the
terms, provisions, covenants and restrictions of this Agreement shall remain in
full force and effect and shall in no way be affected, impaired or invalidated;
provided, however, that notwithstanding anything in this Agreement to the
- --------  -------                                                        
contrary, if any such term, provision, covenant or restriction is held by such
court or authority to be invalid, void or unenforceable and the Board of
Directors of the Company determines in its good faith judgment that severing the
invalid language from this Agreement would adversely affect the purpose or
effect of this Agreement, the right of redemption set forth in Section 23 hereof
shall be reinstated and shall not expire until the close of business on the
tenth day following the date of such determination by the Board of Directors.

      Section 32.   Governing Law.  This Agreement and each Right and each
                    -------------                                         
Rights Certificate issued hereunder shall be deemed to be a contract made under
the laws of the State of Delaware and for all purposes shall be governed by and
construed in accordance with the laws of such State applicable to contracts to
be made and performed entirely within such State.

      Section 33.   Counterparts.  This Agreement may be executed in any number
                    ------------                                               
of counterparts and each of such counterparts shall for all purposes be deemed
to be an original, and all such counterparts shall together constitute but one
and the same instrument.

      Section 34.   Descriptive Headings.  Descriptive headings of the several
                    --------------------                                      
Sections of this Agreement are inserted for convenience only and shall not
control or affect the meaning or construction of any of the provisions hereof.

                                       36
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed as of the day and year first above written.

Attest:                               NATURAL MICROSYSTEMS CORPORATION


By:                                   By:
   -------------------------------       ---------------------------------

Name:                                 Name:
     -----------------------------         -------------------------------

Title:                                Title:
      ----------------------------          ------------------------------


Attest:                               STATE STREET BANK AND TRUST COMPANY


By:                                   By:
   --------------------------------      ---------------------------------

Name:                                 Name:
     ------------------------------        ------------------------------- 

Title:                                Title:
      -----------------------------         ------------------------------

                                       37
<PAGE>
 
                                   EXHIBIT A
                                   ---------

                CERTIFICATE OF DESIGNATIONS, RIGHTS, PREFERENCES
                               AND PRIVILEGES OF

                        SERIES A JUNIOR PREFERRED STOCK

                                       OF

                        NATURAL MICROSYSTEMS CORPORATION

       (Pursuant to Section 151 of the Delaware General Corporation Law)


     The undersigned, Robert P. Schechter and Dianne Callan, do hereby certify:

     1.   That they are the duly elected and acting Chief Executive Officer and
Secretary, respectively, of Natural MicroSystems Corporation, a Delaware
corporation (the "Corporation").

     2.   That pursuant to the authority conferred upon the Board of Directors
by the Certificate of Incorporation of the said Corporation, the said Board of
Directors on December 23, 1998 adopted the following resolutions creating a
series of shares of Preferred Stock designated as Series A Junior Preferred
Stock:

          "RESOLVED, that pursuant to the authority vested in the Board of
Directors of the Corporation by the Certificate of Incorporation, the Board of
Directors does hereby provide for the issue of a series of Preferred Stock,
$0.05 par value, of the Corporation, to be designated "Series A Junior Preferred
Stock," initially consisting of 15,000 shares and to the extent that the
designations, powers, preferences and relative and other special rights and the
qualifications, limitations and restrictions of the Series A Junior Preferred
Stock are not stated and expressed in the Certificate of Incorporation, does
hereby fix and herein state and express such designations, powers, preferences
and relative and other special rights and the qualifications, limitations and
restrictions thereof, as follows (all terms used herein which are defined in the
Certificate of Incorporation shall be deemed to have the meanings provided
therein):

     Section 1.     Designation and Amount.  The shares of such series shall be
                    ----------------------                                     
designated as "Series A Junior Preferred Stock," par value $0.05 per share, and
the number of shares constituting such series shall be 15,000.

     Section 2.     Dividends and Distributions.
                    --------------------------- 

          (A) Subject to the prior and superior right of the holders of any
shares of any series of Preferred Stock ranking prior and superior to the shares
of Series A Junior Preferred Stock with respect to dividends, the holders of
shares of Series A Junior Preferred 
<PAGE>
 
Stock shall be entitled to receive when, as and if declared by the Board of
Directors out of funds legally available for the purpose, quarterly dividends
payable in cash on the last day of March, June, September and December in each
year (each such date being referred to herein as a "Quarterly Dividend Payment
Date") commencing on the first Quarterly Dividend Payment Date after the first
issuance of a share or fraction of a share of Series A Junior Preferred Stock,
in an amount per share (rounded to the nearest cent) equal to, subject to the
provision for adjustment hereinafter set forth, 1,000 times the aggregate per
share amount of all cash dividends, and 1,000 times the aggregate per share
amount (payable in kind) of all non-cash dividends or other distributions other
than a dividend payable in shares of Common Stock or a subdivision of the
outstanding shares of Common Stock (by reclassification or otherwise), declared
on the Common Stock of the Corporation (the "Common Stock") since the
immediately preceding Quarterly Dividend Payment Date, or, with respect to the
first Quarterly Dividend Payment Date, since the first issuance of any share or
fraction of a share of Series A Junior Preferred Stock. In the event the
Corporation shall at any time after December 23, 1998 (the "Rights Declaration
Date") (i) declare any dividend on Common Stock payable in shares of Common
Stock, (ii) subdivide the outstanding Common Stock, or (iii) combine the
outstanding Common Stock into a smaller number of shares, then in each such case
the amount to which holders of shares of Series A Junior Preferred Stock were
entitled immediately prior to such event under the preceding sentence shall be
adjusted by multiplying such amount by a fraction, the numerator of which is the
number of shares of Common Stock outstanding immediately after such event and
the denominator of which is the number of shares of Common Stock that ere
outstanding immediately prior to such event.

          (B) The Corporation shall declare a dividend or distribution on the
Series A Junior Preferred Stock as provided in paragraph (A) above immediately
after it declares a dividend or distribution on the Common Stock (other than a
dividend payable in shares of Common Stock).

          (C) Dividends shall begin to accrue and be cumulative on outstanding
shares of Series A Junior Preferred Stock from the Quarterly Dividend Payment
Date next preceding the date of issue of such shares of Series A Junior
Preferred Stock, unless the date of issue of such shares is prior to the record
date for the first Quarterly Dividend Payment Date, in which case dividends on
such shares shall begin to accrue from the date of issue of such shares, or
unless the date of issue is a Quarterly Dividend Payment Date or is a date after
the record date for the determination of holders of shares of Series A Junior
Preferred Stock entitled to receive a quarterly dividend and before such
Quarterly Dividend Payment Date, in either of which events such dividends shall
begin to accrue and be cumulative from such Quarterly Dividend Payment Date.
Accrued but unpaid dividends shall not bear interest.  Dividends paid on the
shares of Series A Junior Preferred Stock in an amount less than the total
amount of such dividends at the time accrued and payable on such shares shall be
allocated pro rata on a share-by-share basis among all such shares at the time
outstanding. The Board of Directors may fix a record date for the determination
of holders of shares of Series A Junior Preferred Stock entitled to receive
payment of a dividend or distribution declared thereon, which record date shall
be no more than 30 days prior to the date fixed for the payment thereof.

                                       2
<PAGE>
 
     Section 3.     Voting Rights.  The holders of shares of Series A Junior
                    -------------                                           
Preferred Stock shall have the following voting rights.

          (A) Subject to the provision for adjustment hereinafter set forth,
each share of Series A Junior Preferred Stock shall entitle the holder thereof
to 1,000 votes on all matters submitted to a vote of the stockholders of the
Corporation.  In the event the Corporation shall at any time after the Rights
Declaration Date (i) declare any dividend on Common Stock payable in shares of
Common Stock, (ii) subdivide the outstanding Common Stock, or (iii) combine the
outstanding Common Stock to a smaller number of shares, then in each such case
the number of votes per share to which holders of shares of Series A Junior
Preferred Stock were entitled immediately prior to such event shall be adjusted
by multiplying such number by a fraction, the numerator of which is the number
of shares of Common Stock outstanding immediately after such event and the
denominator of which is the number of shares of Common Stock that were
outstanding immediately prior to such event.

          (B) Except as otherwise provided herein or by law, the holders of
shares of Series A Junior Preferred Stock and the holders of shares of Common
Stock shall vote together as one class on all matters submitted to a vote of
stockholders of the corporation.

          (C) Except as required by law, holders of Series A Junior Preferred
Stock shall have no special voting rights and their consent shall not be
required (except to the extent they are entitled to vote with holders of Common
Stock as set forth herein) for taking any corporate action.

     Section 4.     Certain Restrictions.
                    -------------------- 

          (A) The Corporation shall not declare any dividend on, make any
distribution on, or redeem or purchase or otherwise acquire for consideration
any shares of Common Stock after the first issuance of a share or fraction of a
share of Series A Junior Preferred Stock unless concurrently therewith it shall
declare a dividend on the Series A Junior Preferred Stock as required by Section
2 hereof.

          (B) Whenever quarterly dividends or other dividends or distributions
payable on the Series A Junior Preferred Stock as provided in Section 2 are in
arrears, thereafter and until all accrued and unpaid dividends and
distributions, whether or not declared, on shares of Series A Junior Preferred
Stock outstanding shall have been paid in full, the Corporation shall not

               (i) declare or pay dividends on, make any other distributions 
on, or redeem or purchase or otherwise acquire for consideration any shares of
stock ranking junior (either as to dividends or upon liquidation, dissolution or
winding up) to the Series A Junior Preferred Stock;

                                       3
<PAGE>
 
               (ii) declare or pay dividends on or make any other distributions
on any shares of stock ranking on a parity (either as to dividends or upon
liquidation, dissolution or winding up) with Series A Junior Preferred Stock,
except dividends paid ratably on the Series A Junior Preferred Stock and all
such parity stock on which dividends are payable or in arrears in proportion to
the total amounts to which the holders of all such shares are then entitled;

               (iii) redeem or purchase or otherwise acquire for consideration
shares of any stock ranking on a parity (either as to dividends or upon
liquidation, dissolution or winding up) with the Series A Junior Preferred
Stock, provided that the Corporation may at any time redeem, purchase or
       -------- ----                                                    
otherwise acquire shares of any such parity stock in exchange for shares of any
stock of the Corporation ranking junior (either as to dividends or upon
dissolution, liquidation or winding up) to the Series A Junior Preferred Stock;

               (iv) purchase or otherwise acquire for consideration any shares
of Series A Junior Preferred Stock, or any shares of stock ranking on a parity
with the Series A Junior Preferred Stock, except in accordance with a purchase
offer made in writing or by publication (as determined by the Board of
Directors) to all holders of such shares upon such terms as the Board of
Directors, after consideration of the respective annual dividend rates and other
relative rights and preferences of the respective series and classes, shall
determine in good faith will result in fair and equitable treatment among the
respective series or classes.

          (C) The Corporation shall not permit any subsidiary of the Corporation
to purchase or otherwise acquire for consideration any shares of stock of the
Corporation unless the Corporation could, under paragraph (A) of this Section 4,
purchase or otherwise acquire such shares at such time and in such manner.

     Section 5.     Reacquired Shares.  Any shares of Series A Junior Preferred
                    -----------------                                          
Stock purchased or otherwise acquired by the Corporation in any manner
whatsoever shall be retired and cancelled promptly after the acquisition
thereof.  All such shares shall upon their cancellation become authorized but
unissued shares of Preferred Stock and may be reissued as part of a new series
of Preferred Stock to be created by resolution or resolutions of the Board of
Directors, subject to the conditions and restrictions on issuance set forth
herein.

     Section 6.     Liquidation, Dissolution or Winding Up.
                    -------------------------------------- 

          (A) Upon any liquidation (voluntary or otherwise), dissolution or
winding up of the Corporation, no distribution shall be made to the holders of
shares of stock ranking junior (either as to dividends or upon liquidation,
dissolution or winding up) to the Series A Junior Preferred Stock unless, prior
thereto, the holders of shares of Series A Junior Preferred Stock shall have
received an amount equal to accrued and unpaid dividends and distributions
thereon, whether or not declared, to the date of such payment, plus an amount
equal to the greater of (1)$1,000 per share, provided that in the event the
                                             -------- ----                 
Corporation does not have sufficient assets, after payment of its liabilities
and distribution to holders of 

                                       4
<PAGE>
 
Preferred Stock ranking prior to the Series A Junior Preferred Stock, available
to permit payment in full of the $1,000 per share amount, the amount required to
be paid under this Section 6(A)(1) shall, subject to Section 6(B) hereof, equal
the value of the amount of available assets divided by the number of outstanding
shares of Series A Junior Preferred Stock or (2) subject to the provisions for
adjustment hereinafter set forth, 1,000 times the aggregate per share amount to
be distributed to the holders of Common Stock (the greater of (1) or (2), the
"Series A Liquidation Preference"). In the event the Corporation shall at any
time after the Rights Declaration Date (i) declare any dividend on Common Stock
payable in shares of Common Stock, (ii) subdivide the outstanding Common Stock,
or (iii) combine the outstanding Common Stock into a smaller number of shares,
then in each such case the amount to which holders of shares of Series A Junior
Preferred Stock were entitled immediately prior to such event under clause (2)
of the preceding sentence shall be adjusted by multiplying such amount by a
fraction the numerator of which is the number of shares of Common Stock that
were outstanding immediately after such event and the denominator of which is
the number of shares of Common Stock that were outstanding immediately prior to
such event.

          (B) In the event, however, that there are not sufficient assets
available to permit payment in full of the Series A Liquidation Preference and
the liquidation preferences of all other series of Preferred Stock, if any,
which rank on a parity with the Series A Junior Preferred Stock, then such
remaining assets shall be distributed ratably to the holders of such parity
shares in proportion to their respective liquidation preferences.

     Section 7.     Consolidation, Merger, etc.  In case the Corporation shall
                    ---------------------------                               
enter into any consolidation, merger, combination or other transaction in which
the shares of Common Stock are exchanged for or changed into other stock or
securities, cash and/or any other property, then in any such case the shares of
Series A Junior Preferred Stock shall at the same time be similarly exchanged or
changed in an amount per share (subject to the provision for adjustment
hereinafter set forth) equal to 1,000 times the aggregate amount of stock,
securities, cash and/or any other property (payable in kind), as the case may
be, into which or for which each share of Common Stock is changed or exchanged.
In the event the Corporation shall at any time after the Rights Declaration Date
(i) declare any dividend on Common Stock payable in shares of Common Stock, (ii)
subdivide the outstanding Common Stock, or (iii) combine the outstanding Common
Stock into a smaller number of shares, then in each such case the amount set
forth in the preceding sentence with respect to the exchange or change of shares
of Series A Junior Preferred Stock shall be adjusted by multiplying such amount
by a fraction the numerator of which is the number of shares of Common Stock
outstanding immediately after such event and the denominator of which is the
number of shares of Common Stock that were outstanding immediately prior to such
event.

     Section 8.     No Redemption.  The shares of Series A Junior Preferred
                    -------------                                          
Stock shall not be redeemable.

                                       5
<PAGE>
 
     Section 9.     Ranking.  The Series A Junior Preferred Stock shall rank
                    -------                                                 
junior to all other series of the Corporation's Preferred Stock as to the
payment of dividends and the distribution of assets, unless the terms of any
such series shall provide otherwise.

     Section 10.    Amendment.  The Certificate of Incorporation of the
                    ---------                                          
Corporation shall not be further amended in any manner which would materially
alter or change the powers, preference or special rights of the Series A Junior
Preferred Stock so as to affect them adversely without the affirmative vote of
the holders of a majority or more of the outstanding shares of Series A Junior
Preferred Stock, voting separately as a class.

     Section 11.    Fractional Shares.  Series A Junior Preferred Stock may be
                    -----------------                                         
issued in fractions of a share which shall entitle the holder, in proportion to
such holder's fractional shares, to exercise voting rights, receive dividends,
participate in distributions and to have the benefit of all other rights of
holders of Series A Junior Preferred Stock.

     RESOLVED FURTHER, that the Chief Executive Officer, the President or any
Vice President and the Secretary or any Assistant Secretary of this Corporation
be, and they hereby are, authorized and directed to prepare and file a
Certificate of Designations, Rights, Preferences and Privileges in accordance
with the foregoing resolution and the provisions of Delaware law and to take
such actions as they may deem necessary or appropriate to carry out the intent
of the foregoing resolution."

     3.   That the authorized number of shares of Preferred Stock of the
Corporation is 3,000,000 and that no such Preferred Stock has been issued.


     IN WITNESS WHEREOF, we have executed and subscribed this Certificate and do
affirm the foregoing as true under the penalties of perjury this _____ day of
__________, 1998.


                                    --------------------------------------
                                    Name:
                                    Title:

Attest:

 
- ---------------------------------- 
Name:
Title:

                                       6 
<PAGE>
 
                                   EXHIBIT B
                                   ---------

                           Form of Rights Certificate


Certificate No. ___.                                   ____ Rights


NOT EXERCISABLE AFTER DECEMBER 23, 2008 OR EARLIER IF TERMINATED BY THE COMPANY
OR IF THE COMPANY EXCHANGES THE RIGHTS PURSUANT TO THE RIGHTS AGREEMENT.  THE
RIGHTS ARE SUBJECT TO REDEMPTION, AT THE OPTION OF THE COMPANY, AT $0.01 PER
RIGHT ON THE TERMS SET FORTH IN THE RIGHTS AGREEMENT.  UNDER CERTAIN
CIRCUMSTANCES, RIGHTS BENEFICIALLY OWNED BY AN ACQUIRING PERSON OR AN AFFILIATE
OR ASSOCIATE OF AN ACQUIRING PERSON (AS SUCH TERMS ARE DEFINED IN THE RIGHTS
AGREEMENT) AND ANY SUBSEQUENT HOLDER OF SUCH RIGHTS MAY BECOME NULL AND VOID.
[THE RIGHTS REPRESENTED BY THIS RIGHTS CERTIFICATE ARE OR WERE BENEFICIALLY
OWNED BY A PERSON WHO WAS OR BECAME AN ACQUIRING PERSON OR AN AFFILIATE OR
ASSOCIATE OF AN ACQUIRING PERSON (AS SUCH TERMS ARE DEFINED IN THE RIGHTS
AGREEMENT).  ACCORDINGLY, THIS RIGHTS CERTIFICATE AND THE RIGHTS REPRESENTED
HEREBY MAY BECOME NULL AND VOID IN THE CIRCUMSTANCES SPECIFIED IN SECTION 7(e)
OF SUCH RIGHTS AGREEMENT.]/1/



                               Rights Certificate


     This certifies that ____________, or registered assigns, is the registered
owner of the number of Rights set forth above, each of which entitles the owner
thereof, subject to the terms, provisions and conditions of the Rights
Agreement, dated as of December 23, 1998 (the "Rights Agreement"), between
Natural MicroSystems Corporation, a Delaware corporation (the "Company"), and
State Street Bank and Trust Company (the "Rights Agent"), to purchase from the
Company at any time after the Distribution Date (as such term is defined in the
Rights Agreement) and prior to 5:00 P.M., Massachusetts time, on December 23,
2008 at the office of the Rights Agent designated for such purpose, or at the
office of its successor as Rights Agent, one one-thousandth of a fully paid non-
assessable share of Series A Junior Preferred Stock, par value $0.05 per share,
(the "Preferred Shares"), of the Company, at a purchase price of $______ [Insert
the Purchase Price as of the Distribution Date] per one-thousandth of a
Preferred Share (the "Purchase Price"), upon presentation and surrender of this
Rights Certificate with the Form of Election to Purchase and related Certificate
duly executed.  The number of Rights evidenced by this Rights 


- ------------------------------
/1/ The portion of the legend in brackets shall be inserted only if
applicable and shall replace the preceding sentence.
<PAGE>
 
Certificate (and the number of one-thousandths of a Preferred Share which may be
purchased upon exercise hereof) set forth above are the number and Purchase
Price as of ____________ [Insert the Distribution Date] based on the Preferred
Shares as constituted at such date. As provided in the Rights Agreement, the
Purchase Price and the number and kind of Preferred Shares or other securities
which may be purchased upon the exercise of the Rights evidenced by this Rights
Certificate are subject to modification and upon the happening of certain
events.

     This Rights Certificate is subject to all of the terms, provisions and
conditions of the Rights Agreement, which terms, provisions and conditions are
hereby incorporated herein by reference and made a part hereof and to which
Rights Agreement reference is hereby made for a full description of the rights,
limitations of rights, obligations, duties and immunities hereunder of the
Rights Agent, the Company and the holders of the Rights Certificates, which
limitations of rights include the temporary suspension of the exercisability of
such Rights under the specific circumstances set forth in the Rights Agreement.
Copies of the Rights Agreement are on file at the principal executive offices of
the Company and the above-mentioned office of the Rights Agent.

     Subject to the provisions of the Rights Agreement, the Rights evidenced by
this Rights Certificate (i) may be redeemed by the Company, at its option, at a
redemption price of $0.01 per Right or (ii) may be exchanged by the Company in
whole or in part for Common Shares, substantially equivalent rights or other
consideration as determined by the Company.

     This Rights Certificate, with or without other Rights Certificates, upon
surrender at the office of the Rights Agent designated for such purpose, may be
exchanged for another Rights Certificate or Rights Certificates of like tenor
and date evidencing Rights entitling the holder to purchase a like aggregate
amount of securities as the Rights evidenced by the Rights Certificate or Rights
Certificates surrendered shall have entitled such holders to purchase.  If this
Rights Certificate shall be exercised in part, the holder shall be entitled to
receive upon surrender hereof another Rights Certificate or Rights Certificates
for the number of whole Rights not exercised.

     No fractional portion of less than one one-thousandth of a Preferred Share
will be issued upon the exercise of any Right or Rights evidenced hereby but in
lieu thereof a cash payment will be made, as provided in the Rights Agreement.

     No holder of this Rights Certificate, as such, shall be entitled to vote or
receive dividends or be deemed for any purpose the holder of the Preferred
Shares or of any other securities of the Company which may at any time be
issuable on the exercise hereof, nor shall anything contained in the Rights
Agreement or herein be construed to confer upon the holder hereof, as such, any
of the rights of a stockholder of the Company or any right to vote for the
election of directors or upon any matter submitted to stockholders at any
meeting thereof, or to give or withhold consent to any corporate action, or to
receive notice of 

                                       2
<PAGE>
 
meetings or other actions affecting stockholders (except as provided in the
Rights Agreement), or to receive dividends or subscription rights, or otherwise,
until the Right or Rights evidenced by this Rights Certificate shall have been
exercised as provided in the Rights Agreement.

     This Rights Certificate shall not be valid or obligatory for any purpose
until it shall have been countersigned by the Rights Agent.

     WITNESS the facsimile signature of the proper officers of the Company and
its corporate seal.  Dated as of _______________ ___, ____.

ATTEST                              NATURAL MICROSYSTEMS CORPORATION


                                    By:
- --------------------------------       ----------------------------------
                                       Chief Executive Officer
- --------------------------------
Secretary                       


Countersigned:


STATE STREET BANK AND
TRUST COMPANY,
as Rights Agent


By:
   -----------------------------
   Authorized Signature

                                       3
<PAGE>
 
                   Form of Reverse Side of Rights Certificate

                               FORM OF ASSIGNMENT
                               ------------------

                (To be executed by the registered holder if such
               holder desires to transfer the Rights Certificate)


     FOR VALUE RECEIVED ______________________________ hereby sells, assigns and
transfers unto

__________________________________________________________________
(Please print name and address of transferee)

this Rights Certificate, together with all right, title and interest therein,
and does hereby irrevocably constitute and appoint ___________________________
Attorney, to transfer the within Rights Certificate on the books of the within-
named Company, with full power of substitution.



Dated: ___________, ______    ___________________________________
                              Signature



Signature Guaranteed:

     The signature(s) should be guaranteed by an eligible guarantor institution
(Banks, Stockbrokers, Savings and Loan Associations and Credit Unions with
membership in an approved signature guarantee medallion program) pursuant to
S.E.C. Rule 17Ad-15.

                                       4
<PAGE>
 
            Form of Reverse Side of Rights Certificate -- continued

                                  CERTIFICATE
                                  -----------


     The undersigned hereby certifies by checking the appropriate boxes that:

     (1) this Rights Certificate [  ] is [  ] not being sold, assigned and
transferred by or on behalf of a Person who is or was an Acquiring Person, or an
Affiliate or Associate of any such Person (as such terms are defined in the
Rights Agreement);

     (2) after due inquiry and to the best knowledge of the undersigned, it [  ]
did [  ] did not acquire the Rights evidenced by this Rights Certificate from
any Person who is, was or subsequently became an Acquiring Person or an
Affiliate or Associate of any such Person.



Dated: ______________ , _____      _________________________________
                                   Signature

Signature Guaranteed:

     The signature(s) should be guaranteed by an eligible guarantor institution
(Banks, Stockbrokers, Savings and Loan Associations and Credit Unions with
membership in an approved signature guarantee medallion program) pursuant to
S.E.C. Rule 17Ad-15.

                                       5
<PAGE>
 
            Form of Reverse Side of Rights Certificate -- continued

                          FORM OF ELECTION TO PURCHASE
                          ----------------------------

                      (To be executed if holder desires to
                        exercise the Rights Certificate)

To:________________________

     The undersigned hereby irrevocably elects to exercise ____________________
Rights represented by this Rights Certificate to purchase the number of one-
thousandths of a Preferred Share issuable upon the exercise of such Rights and
requests that certificates for such number of one-thousandths of a Preferred
Share issued in the name of:

Please insert social security or
other identifying number

_____________________________________________________________________
                        (Please print name and address)

_____________________________________________________________________

If such number of Rights shall not be all the Rights evidenced by this Rights
Certificate, a new Rights Certificate for the balance remaining of such Rights
shall be registered in the name of and delivered to:

Please insert social security or
other identifying number

_____________________________________________________________________
                        (Please print name and address)

_____________________________________________________________________


Dated: ______________ , ______      _________________________________
                                    Signature

Signature Guaranteed:

     The signature(s) should be guaranteed by an eligible guarantor institution
(Banks, Stockbrokers, Savings and Loan Associations and Credit Unions with
membership in an approved signature guarantee medallion program) pursuant to
S.E.C. Rule 17Ad-15.

                                       6
<PAGE>
 
            Form of Reverse Side of Rights Certificate -- continued

                                  CERTIFICATE
                                  -----------


     The undersigned hereby certifies by checking the appropriate boxes that:

     (1) the Rights evidenced by this Rights Certificate [  ] are [  ] are not
being exercised by or on behalf of a Person who is or was an Acquiring Person or
an Affiliate or Associate of any such Person (as such terms are defined in the
Rights Agreement);

     (2) after due inquiry and to the best knowledge of the undersigned, it [  ]
did [  ] did not acquire the Rights evidenced by this Rights Certificate from
any Person who is, was or subsequently became an Acquiring Person or an
Affiliate or Associate of any such Person.



Dated: ________________, _____      ____________________________
                                    Signature



Signature Guaranteed:

     The signature(s) should be guaranteed by an eligible guarantor institution
(Banks, Stockbrokers, Savings and Loan Associations and Credit Unions with
membership in an approved signature guarantee medallion program) pursuant to
S.E.C. Rule 17Ad-15.

                                       7
<PAGE>
 
            Form of Reverse Side of Rights Certificate -- continued


                                     NOTICE
                                     ------


     The signature in the foregoing Forms of Assignment and Election and
Certificates must conform to the name as written upon the face of this Rights
Certificate in every particular, without alteration or enlargement or any change
whatsoever.

                                       8
<PAGE>
 
                                   EXHIBIT C
                                   ---------

                        NATURAL MICROSYSTEMS CORPORATION
                            STOCKHOLDER RIGHTS PLAN


                               Summary of Rights
                               -----------------


Distribution and    The Board of Directors has declared a dividend of one Right
Transfer of Rights; each share of Natural MicroSystems Corporation Common Stock
Rights Certificate: outstanding.  Prior to the Distribution Date referred to
- ------------------  below, the Rights will be evidenced by and trade with
                    the certificates for the Common Stock.  After the
                    Distribution Date, Natural MicroSystems Corporation (the
                    "Company") will mail Rights certificates to the Company's
                    stockholders and the Rights will become transferable apart
                    from the Common Stock.

Distribution Date:  Rights will separate from the Common Stock and become
- -----------------   exercisable following the tenth day (the "Distribution
                    Date") (or such later date as may be determined by a
                    majority of the Directors not affiliated with the acquiring
                    person or group (the "Continuing Directors")) after a person
                    or group (a) acquires beneficial ownership of 15% or more of
                    the Company's Common Stock or (b) announces a tender or
                    exchange offer, the consummation of which would result in
                    ownership by a person or group of 30% or more of the
                    Company's Common Stock.

Preferred Stock     After the Distribution Date, each Right will entitle the
Purchasable Upon    holder to purchase, for $60.00, a fraction of a share of the
Exercise of Rights: Company's Preferred Stock with economic terms similar to 
- ------------------  that of one share of the Company's Common Stock.

Flip-In:            If an acquiror (an "Acquiring Person") obtains 15% or more
- -------             of the Company's Common Stock (other than pursuant to a
                    tender offer deemed fair by the Board of Directors (a
                    "Permitted Offer")), then each Right (other than Rights
                                         ----                              
                    owned by an Acquiring Person or its affiliates) will entitle
                    the holder thereof to purchase, for the exercise price, a
                    number of shares of the Company's Common Stock having a then
                    current market value of twice the exercise price.

                                       1
<PAGE>
 
Flip-Over:          If, after the Shares Acquisition Date (defined below), (a)
- ---------           the Company merges into another entity, (b) an acquiring 
                    entity merges into the Company or (c) the Company sells more
                    than 50% of the Company's assets or earning power, then each
                    Right (other than Rights owned by an Acquiring Person or its
                    affiliates) will entitle the holder thereof to purchase, for
                    the exercise price, a number of shares of Common Stock of
                    the person engaging in the transaction having a then current
                    market value of twice the exercise price (unless the
                    transaction satisfies certain conditions and is consummated
                    with a person who acquired shares pursuant to a Permitted
                    Offer, in which case the Rights will expire).

Exchange Provision: At any time after an event triggering the flip-in or flip-
- ------------------  over rights and prior to the acquisition by the Acquiring 
                    Person of 50% or more of the outstanding Common Stock, the
                    Board of Directors of the Company may exchange the Rights
                    (other than Rights owned by the Acquiring Person or its
                    affiliates), in whole or in part, at an exchange ratio of
                    one Common Share per Right (subject to adjustment).

Redemption of       Rights will be redeemable at the Company's option for $0.01
the Rights:         per Right at any time on or prior to the tenth day (or such
- ----------          later date as may be determined by a majority of the 
                    Continuing Directors) after public announcement that a
                    person has acquired beneficial ownership of 15% or more of
                    the Company's Common Stock (the "Shares Acquisition Date").

Expiration of       The Rights expire on the earliest of (a) December 23, 2008,
the Rights:         (b) exchange or redemption of the Rights as described 
- ----------          above, or (c) consummation of a merger or consolidation or
                    sale of assets resulting in expiration of the Rights as
                    described above.

Amendment of Terms  The terms of the Rights and the Rights Agreement may be
of Rights:          amended in any respect without the consent of the Rights
- ---------           holders on or prior to the Distribution Date; thereafter, 
                    the terms of the Rights and the Rights Agreement may be
                    amended without the consent of the Rights holders in order
                    to cure any ambiguities or to make changes which do not
                    adversely affect the interests of Rights holders (other than
                    the Acquiring Person).

Voting Rights:      Rights will not have any voting rights.
- -------------                                              

Anti-Dilution       Rights will have the benefit of certain customary 
Provisions:         anti-dilution provisions.
- ----------                                     

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<PAGE>
 
Taxes:              The Rights distribution should not be taxable for federal
- -----               income tax purposes.  However, following an event which
                    renders the Rights exercisable or upon redemption of the
                    Rights, stockholders may recognize taxable income.

The foregoing is a summary of certain principal terms of the Stockholder Rights
Plan only and is qualified in its entirety by reference to the detailed terms of
the Rights Agreement, dated as of December 23, 1998, between the Company and the
Rights Agent.

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