Exhibit 4.1

                   AMENDMENT NO. 1 TO THE RIGHTS AGREEMENT


            This Amendment No. 1 (this "Amendment"), dated as of July 12, 2002,
between PHARMACIA CORPORATION, a Delaware corporation (the "Company") and MELLON
INVESTOR SERVICES LLC, a New Jersey limited liability company, as Rights Agent
(the "Rights Agent"), to the Amended and Restated Rights Agreement, dated as of
February 20, 2001, between the Company and the Rights Agent (the "Rights
Agreement"); all capitalized terms not defined herein shall have the meanings
ascribed to such terms in the Rights Agreement.

            WHEREAS, the Company proposes to enter into an Agreement and Plan of
Merger, dated as of July 13, 2002 (as amended, supplemented, modified or
replaced from time to time, the "Merger Agreement"), among Pfizer, Inc., a
Delaware corporation ("Parent"), Pfizer Acquisition Sub Corp., a Delaware
corporation and a direct, wholly owned subsidiary of Parent ("Merger Sub"), and
the Company;

            WHEREAS, the Board of Directors of the Company has determined that
the Merger Agreement and the terms and conditions set forth therein and the
transactions contemplated thereby, including, without limitation, the Merger (as
defined in the Merger Agreement), are advisable and are fair to and in the best
interests of the Company and its stockholders;

            WHEREAS, the Board of Directors of the Company has determined, in
connection with its contemplation of the Merger Agreement, that it is necessary
and desirable to amend the Rights Agreement to exempt the Merger Agreement and

the transactions contemplated thereby, including, without limitation, the
Merger, from the application of the Rights Agreement as set forth in this
Amendment;

            WHEREAS, Section 27 of the Rights Agreement provides that the
Company may in its sole and absolute discretion from time to time supplement and
amend the Rights Agreement without the approval of any holders of Rights
Certificates in order, among other things, to make any provisions with respect
to the Rights which the Company may deem necessary or desirable; and

            WHEREAS, pursuant to Section 27, the Company hereby directs that the
Rights Agreement should be amended as set forth in this Amendment.

            NOW THEREFORE, in consideration of the foregoing premises and mutual
covenants and agreements set forth herein, and other good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged, the
Company and the Rights Agent hereby agree as follows:

            Section 1. Amendment to Section 1(a). Section 1(a) of the Rights
Agreement is hereby amended and supplemented by adding the following sentence to
the end thereof:

      "Notwithstanding anything in this Agreement to the contrary, neither
      Pfizer, Inc., a Delaware corporation ("Parent"), nor Pfizer Acquisition
      Sub Corp., a Delaware corporation and a direct, wholly owned subsidiary of
      Parent ("Merger Sub"), nor any of Parent's Affiliates shall become or be
      deemed to be an Acquiring Person as a result of (i) the approval,
      execution, delivery or performance of the Agreement and Plan of Merger,
      dated as of July 13, 2002, among Parent, Merger Sub and the Company (as
      amended, supplemented, modified or replaced from time to time, the "Merger
      Agreement"), (ii) the consummation of the Merger (as defined in the Merger
      Agreement), (iii) the consummation of any other transaction contemplated
      in the Merger Agreement, including the exchange of common stock of the
      Company for common stock

      of the Parent thereunder pursuant to the Merger Agreement, or (iv) the
      public announcement of any of the foregoing."

            Section 2. Amendment to Section 1(l). Section 1(l) of the Rights
Agreement is hereby amended and supplemented by adding the following sentence to
the end thereof:

      "Notwithstanding anything in this Agreement to the contrary, a Shares
      Acquisition Date shall not occur or be deemed to have occurred as a result
      of (i) the approval, execution, delivery or performance of the Merger
      Agreement, (ii) the consummation of the Merger, (iii) the consummation of
      any other transaction contemplated in the Merger Agreement, including the
      exchange of common stock of the Company for common stock of the Parent
      thereunder pursuant to the Merger Agreement, or (iv) the public
      announcement of any of the foregoing."

            Section 3. Amendment to Section 3. Section 3 of the Rights Agreement
is hereby amended and supplemented by adding the following proviso to the end of
the first sentence thereof:

      "; provided that notwithstanding anything in this Agreement to the
      contrary, a Distribution Date shall not occur or be deemed to have
      occurred as a result of (i) the approval, execution, delivery or
      performance of the Merger Agreement, (ii) the consummation of the Merger,
      (iii) the consummation of any other transaction contemplated in the Merger
      Agreement, including the exchange of common stock of the Company for
      common stock of the Parent thereunder pursuant to the Merger Agreement, or
      (iv) the public announcement of any of the foregoing."

            Section 4. Effective Date. This Amendment shall be deemed effective
as of the date first written above, as if executed on such date.

            Section 5. Governing Law. This Amendment 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; provided,
however, that all provisions regarding the rights, duties and obligations of the
Rights Agent shall be governed by and construed in accordance with the laws of
the State of New York applicable to contracts made and to be performed entirely
within such State.

            Section 6. Severability. If any term, provision, covenant or
restriction of this Amendment is held by a court of competent jurisdiction or
other competent authority to be invalid, illegal or incapable of being enforced,
the remainder of the terms, provisions, covenants and restrictions of this
Amendment, and of the Rights Agreement, shall remain in full force and effect
and shall in no way be affected, impaired or invalidated. Upon any such
determination that any term or other provision is invalid, illegal or incapable
of being enforced, the parties hereto shall negotiate in good faith to modify
such provision so as to effect the original intent of the parties as closely as
possible and in an acceptable manner with respect to such provision to the
greatest extent possible.

            Section 7. Notice. The Rights Agent and the Company hereby waive any
notice requirement with respect to each other under the Rights Agreement, if
any, pertaining to the matters covered by this Amendment.

            Section 8. No Other Effect. Except as expressly set forth herein,
the Rights Agreement shall not by implication or otherwise be supplemented or
amended by virtue of this Amendment, but shall remain in full force and effect,
as amended hereby.

            Section 9. Counterparts. This Amendment 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.

            IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed as of the date and year first above written.


                                          PHARMACIA CORPORATION

                                          By    /s/ Judith Reinsdorf
                                                -------------------------------
                                                Name: Judith Reinsdorf
                                                Title:Vice President and
                                                      Assistant Secretary

                                          MELLON INVESTOR SERVICES LLC

                                          By    /s/ Daniel M. Egan
                                                -------------------------------
                                                Name: Daniel M. Egan
                                                Title:Vice President