Exhibit (a)(5)

                                 MERRIMAC SERIES
                               AMENDMENT NO. 4 TO
                             MASTER TRUST AGREEMENT

     AMENDMENT NO. 4 to the Master Trust Agreement dated March 30, 1998 of the
Merrimac Series (the "Trust") made at Boston, Massachusetts as of this 7th day
of November, 2002.

     WHEREAS, Section 7.3 of the Agreement of the Trust provides that the
Agreement may be amended at any time by an instrument in writing signed by a
majority of Trustees of the Trust without the vote of the Shareholders of the
Trust, so long as such amendment does not adversely affect the rights of any
shareholder;

     WHEREAS, Section 4.1 of the Agreement of the Trust provides that the
Trustees of the Trust may establish and designate additional Series of Shares by
an instrument in writing signed by a majority of Trustees of the Trust; and

     WHEREAS, the Trustees of the Trust desire to establish an additional Series
of Shares to be identified as the "Merrimac Prime Series."

     NOW, THEREFORE, the Trustees hereby state that:

     1. Section 4.2 of the Agreement and all other appropriate references in the
Agreement are amended to designate and establish a new Series of shares (in
addition to the Merrimac Cash Series, the Merrimac Treasury Series, the Merrimac
Treasury Plus Series, the Merrimac U.S. Government Series and the Merrimac
Municipal Series heretofore established and designated) to be known as the
Merrimac Prime Series, effective as of this date, such new Series to have the
relative rights and preferences set forth in Section 4.2 of the Agreement.

     2. The initial paragraph of Section 4.2 of the agreement, as heretofore in
effect, is amended to read as follows:

     "Section 4.2 Establishment and Designation of Sub-Trusts and Classes.
     Without limiting the authority of the Trustees set forth in Section 4.1 to
     establish and designate any further Sub-Trusts, the Trustees hereby
     establish and designate six Sub-Trusts identified as "Merrimac Cash
     Series," "Merrimac Treasury Series," "Merrimac Treasury Plus Series,"
     "Merrimac U.S. Government Series," " Merrimac Municipal Series" and the
     "Merrimac Prime Series" which Sub-Trusts shall consist of five classes of
     shares identified as the "Premium Class," the "Institutional Class,"
     "Investment Class," "Reserve Class" and "Adviser Class" Shares. The Shares
     of such Sub-Trusts and any Shares of any further Sub-Trusts that may from
     time to time be established and designated by the Trustees shall (unless
     the Trustees otherwise determine with respect to some further Sub-Trust at
     the time of establishing and designating the same) have the following
     relative rights and preferences:"


     IN WITNESS WHEREOF, the undersigned have hereunto set their hands and seal
for themselves and their assigns, as of this 7th day of November, 2002.


/s/Edward F. Hines, Jr.
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Edward F. Hines, Jr.


/s/Francis J. Gaul, Jr.
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Francis J. Gaul, Jr.


/s/Thomas E. Sinton
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Thomas E. Sinton


/s/Kevin J. Sheehan
--------------------------------------------
Kevin J. Sheehan