Exhibit 99.1

         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

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

      1.    The N-CSR of the Fund (the "Report") fully complies with the
            requirements of Section 13(a) or 15(d) of the Securities Exchange
            Act of 1934; and

      2.    The information contained in the Report fairly presents, in all
            material respects, the financial condition and results of operations
            of the Fund.

Dated: February 18, 2003

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



Exhibit 99.1

         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

I, Terry K. Glenn, President of Mercury Low Duration Fund (the "Fund"), certify
that:

      1.    The N-CSR of the Fund (the "Report") fully complies with the
            requirements of Section 13(a) or 15(d) of the Securities Exchange
            Act of 1934; and

      2.    The information contained in the Report fairly presents, in all
            material respects, the financial condition and results of operations
            of the Fund.

Dated: February 18, 2003

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