Exhibit 5.1



                                                                 August 28, 2003


Wyeth
Five Giralda Farms
Madison, New Jersey 07940

Ladies and Gentlemen:
      We have acted as counsel to Wyeth, a Delaware corporation (the "Company"),
in connection with the Registration Statement on Form S-3 (the "Registration
Statement") filed by the Company with the Securities and Exchange Commission
(the "Commission") under the Securities Act of 1933, as amended, relating to the
issuance by the Company of up to $2,500,000,000 aggregate principal amount of
debt securities (the "Securities"). The Securities will be issued under the
indenture (the "Indenture") dated as of April 19, 1992, as amended on October
13, 1992, between the Company and JPMorgan Chase Bank (successor to The Chase
Manhattan Bank), as Trustee, as the same may be supplemented by a supplemental
indenture in relation to the offering of particular Securities.

      We have examined the Registration Statement and the Indenture, which has
been filed with the Commission as an exhibit to the Registration Statement. We
also have examined the originals, or duplicates or certified or conformed
copies, of such records, agreements, instruments and other documents and have
made such other and further investigations as we have deemed relevant and
necessary in connection with the opinions expressed herein. As to questions of
fact material to this opinion, we have relied upon certificates of public
officials and of officers and representatives of the Company.

      In rendering the opinion set forth below, we have assumed the genuineness
of all signatures, the legal capacity of natural persons, the authenticity of
all documents submitted to us as originals, the conformity to original documents
of all documents submitted to us as duplicates or certified or conformed copies,
and the authenticity of the originals of such latter documents. We also have
assumed that the Indenture is the valid and legally binding obligation of the
Trustee.

      Based upon the foregoing, and subject to the qualifications and
limitations stated herein, we are of the opinion that:

      Assuming (a) the taking of all necessary corporate action to approve the
issuance and terms of any Securities, the terms of the offering thereof and
related matters by the Board of Directors of the Company, a duly constituted and
acting committee of such Board or duly authorized officers of the Company (such
Board of Directors, committee or authorized officers being hereinafter referred
to as the "Board") and (b) the due execution, authentication, issuance and
delivery of such Securities, upon payment of the consideration therefor provided
for in the applicable definitive purchase, underwriting or similar agreement
approved by the Board and otherwise in accordance with the provisions of the
Indenture and such agreement, such Securities will constitute valid and legally
binding obligations of the Company enforceable against the Company in accordance
with their terms.

      Our opinion set forth above is subject to the effects of (i) bankruptcy,
insolvency, fraudulent conveyance, reorganization, moratorium and other similar
laws relating to or affecting creditors' rights generally, (ii) general
equitable principles (whether considered in a proceeding in equity or at law)
and (iii) an implied covenant of good faith and fair dealing.

      We are members of the Bar of the State of New York, and we do not express
any opinion herein concerning any law other than the law of the State of New
York, the Federal law of the United States and the Delaware General Corporation
Law (including the statutory provisions, all applicable provisions of the
Delaware Constitution and reported judicial decisions interpreting the
foregoing).

      We hereby consent to the filing of this opinion letter as Exhibit 5.1 to
the Registration Statement and to the use of our name under the caption "Legal
Matters" in the Prospectus included in the Registration Statement.

                                          Very truly yours,
                                          SIMPSON THACHER & BARTLETT LLP