EX-99. CERT

                     CERTIFICATION PURSUANT TO SECTION 302
                        OF THE SARBANES-OXLEY ACT OF 2002

--------------------------------------------------------------------------------

I, Donald C. Burke, Chief Financial Officer of Mercury Low Duration Fund,
certify that:

      1.    I have reviewed this report on Form N-CSR of Mercury Low Duration
            Fund and Master Low Duration Bond Fund;

      2.    Based on my knowledge, this report does not contain any untrue
            statement of a material fact or omit to state a material fact
            necessary to make the statements made, in light of the circumstances
            under which such statements were made, not misleading with respect
            to the period covered by this report; and

      3.    Based on my knowledge, the financial statements, and other financial
            information included in this report, fairly present in all material
            respects the financial condition, results of operations, changes in
            net assets, and cash flows (if the financial statements are required
            to include a statement of cash flows) of the registrant as of, and
            for, the periods presented in this report.

      Date: February 18, 2003

                                                      /s/ Donald C. Burke
                                                      --------------------------
                                                      Donald C. Burke,
                                                      Chief Financial Officer of
                                                      Mercury Low Duration Fund


EX-99. CERT

                      CERTIFICATION PURSUANT TO SECTION 302
                        OF THE SARBANES-OXLEY ACT OF 2002

--------------------------------------------------------------------------------

I, Terry K. Glenn, President of Mercury Low Duration Fund, certify that:

      1.    I have reviewed this report on Form N-CSR of Mercury Low Duration
            Fund and Master Low Duration Bond Fund;

      2.    Based on my knowledge, this report does not contain any untrue
            statement of a material fact or omit to state a material fact
            necessary to make the statements made, in light of the circumstances
            under which such statements were made, not misleading with respect
            to the period covered by this report; and

      3.    Based on my knowledge, the financial statements, and other financial
            information included in this report, fairly present in all material
            respects the financial condition, results of operations, changes in
            net assets, and cash flows (if the financial statements are required
            to include a statement of cash flows) of the registrant as of, and
            for, the periods presented in this report.

      Date: February 18, 2003

                                                       /s/ Terry K. Glenn
                                                       -------------------------
                                                       Terry K. Glenn,
                                                       President of
                                                       Mercury Low Duration Fund