Exhibit 5.1

                                                               December 11, 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") pursuant to Rule 462(b) under the Securities Act of 1933, as
amended (the "Act") relating to the issuance by the Company of up to
$500,000,000 of debt securities (the "Securities"). The Registration Statement
incorporates by reference in its entirety the Company's Registration Statement
on Form S-3 (File No. 333-108312) (the "Original Registration Statement")
declared effective on November 24, 2003 by the Commission. The Securities will
be issued under a supplemental indenture to the Indenture dated as of April 19,
1992, as amended on October 13, 1992 (the "Indenture"), between the Company and
JPMorgan Chase Bank (successor to The Chase Manhattan Bank), as trustee (the
"Trustee").

      We have examined the Registration Statement, the Original Registration
Statement, and the Indenture, which has been filed with the Commission as an
exhibit to the Original Registration Statement. We also have examined the
originals, or duplicates or certified or conformed copies, of such corporate
records, agreements, documents and other instruments and have made such other
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

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certificates or comparable documents 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 referred to herein 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,
the 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 (1) bankruptcy,
insolvency, fraudulent conveyance, reorganization, moratorium and other similar
laws relating to or affecting creditors' rights generally, (2) general equitable
principles (whether considered in a proceeding in equity or at law) and (3) 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


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                                                               December 11, 2003

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 and prospectus supplement incorporated by reference
into the Registration Statement.

                                        Very truly yours,


                                        SIMPSON THACHER & BARTLETT LLP


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