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                                                                     EXHIBIT 5.1


                       OPINION OF MORRISON & FOERSTER LLP



                                 July 20, 2000



Connetics Corporation
3400 West Bayshore Road
Palo Alto, CA  94303

Ladies and Gentlemen:

     At your request, we have examined the Registration Statement on Form S-3
(File No. 333-41048) (the "Registration Statement"), filed with the Securities
and Exchange Commission under the Securities Act of 1933, as amended (the
"Act"), relating to 2,010,000 shares of your common stock, $.001 par value per
share (the "Shares"). In connection therewith, we have reviewed certain of your
corporate records and proceedings taken in connection with the authorization and
issuance of the Shares, and such other factual and legal matters as we have
considered necessary for purposes of this opinion.

     Based on and subject to the foregoing, we are of the opinion that the
Shares are legally issued, fully paid and nonassessable.

     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to this firm under the heading
"Legal Matters" in such Registration Statement. This consent does not constitute
a consent under Section 7 of the Act, and in consenting to the reference to our
firm under such heading we have not certified any part of the Registration
Statement and do not otherwise come within the categories of persons whose
consent is required under Section 7 or the rules and regulations of the
Securities and Exchange Commission thereunder.

                                       Very truly yours,

                                       /s/ MORRISON & FOERSTER LLP
                                       -------------------------------
                                       Morrison & Foerster LLP