EXHIBIT 5.1

                              OPINION OF COUNSEL

October 2, 2000
Mercury Interactive Corporation
1325 Borregas Avenue
Sunnyvale, California 94089

         Re: Registration Statement on Form S-8
Ladies and Gentlemen:

     We have examined the Registration  Statement on Form S-8 to be filed by
you with the Securities and Exchange Commission on or about October 2, 2000 (the
"Registration   Statement")  in  connection  with  the  registration  under  the
Securities  Act of 1933,  as  amended,  of a total of  2,000,000  shares of your
Common Stock (the "Shares")  reserved for issuance  under the 2000  Supplemental
Stock  Option  Plan (the  "Plan").  As legal  counsel  for  Mercury  Interactive
Corporation,  we have examined the  proceedings  taken and are familiar with the
proceedings proposed to be taken by you in connection with the sale and issuance
of the Shares under the Plan.

     It is our opinion that, when issued and sold in the manner referred to in
the Plan and pursuant to the agreement which accompanies each grant under the
Plan, the Shares will be legally and validly issued, fully paid and
nonassessable.

     We consent to the use of this opinion as an exhibit to the Registration
Statement and further  consent to the use of our name wherever it appears in the
Registration Statement and any amendments thereto.

                                   Very truly yours,

                                   GENERAL COUNSEL ASSOCIATES LLP

                                   /s/ General Counsel Associates LLP