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                                                                     Exhibit 5.1


                     [LETTERHEAD OF SULLIVAN & CROMWELL]




                                                             June 26, 2000



Pharmacia Corporation,
   100 Route 206 North,
      Peapack, New Jersey  07977.

Ladies and Gentlemen:

                  In connection with the registration under the Securities Act
of 1933, as amended (the "Act"), of 400,000 shares of Common Stock, par value
$2.00 per share (the "Shares"), of Pharmacia Corporation, a Delaware corporation
(the "Company"), issuable upon the exercise of certain options granted pursuant
to the Pharmacia Corporation Directors Equity Compensation and Deferral Plan
(the "Plan"), and the related preferred stock purchase rights (the "Rights") to
be issued pursuant to the Rights Agreement, dated as of December 19, 1999 (the
"Rights Agreement"), between the Company and Equiserve Trust Company N.A., as
Rights Agent (the "Rights Agent"), we, as your counsel, have examined such
corporate records, certificates and other documents, and such questions of law,
as we have considered necessary or appropriate for the purposes of this opinion.
Upon the basis of such examination, we advise you that, in our opinion:

     (1) Shares issued pursuant to Stock Awards granted pursuant to the Plan
will be validly issued, fully paid and nonassessable.

     (2) When the related options have been duly exercised in accordance with
the Plan and the related option agreements and the exercise price therefor has
been duly paid, Shares issued upon exercise of options granted pursuant to the
Plan will be validly issued, fully paid and nonassessable.

     (3) Assuming that the Board of Directors of the Company, after fully
informing itself with respect to the Rights Agreement and the Rights after
giving due consideration to all relevant matters, determined that the execution
and delivery of the Rights thereunder would be in the best interest of the
Company and its stockholders, and assuming further that the Rights Agreement has
been duly authorized, executed and delivered by the Rights Agent, then when
Shares are issued pursuant to Stock Awards granted pursuant to the Plan or upon
the exercise of options granted pursuant to the Plan, the Rights attributable to
the Shares will be validly issued.
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                 In connection with our opinion set forth in paragraph (3)
above, we note that the question whether the Board of Directors of the Company
might be required to redeem the Rights at some future time will depend upon
facts and circumstances existing at that time and, accordingly, is beyond the
scope of such opinion.

                 The foregoing opinion is limited to the Federal laws of the
United States and the General Corporation Law of the State of Delaware, and we
are expressing no opinion as to the effect of the laws of any other
jurisdiction.
                  We have relied as to certain matters on information obtained
from public officials, officers of the Company and other sources believed by us
to be responsible.

                  We hereby consent to the filing of this opinion as an exhibit
to the Registration Statement relating to the Shares and the Rights. In giving
such consent, we do not thereby admit that we are in the category of persons
whose consent is required under Section 7 of the Act.

                                                    Very truly yours,

                                                    /s/ Sullivan & Cromwell