Exhibit 4(b)

                        SUB-INVESTMENT ADVISORY AGREEMENT

      AGREEMENT dated September 29, 2006,  between  BlackRock  Advisors,  LLC, a
Delaware limited liability company (the "Advisor"),  and BlackRock Institutional
Management Corporation, a Delaware corporation (the "Sub-Advisor").

      WHEREAS, the Advisor has agreed to furnish investment advisory services to
Master Government Securities Trust, a Delaware statutory trust (the "Trust"), an
open-end  management  investment company registered under the Investment Company
Act of 1940, as amended (the "1940 Act");

      WHEREAS,  the Advisor wishes to retain the  Sub-Advisor to provide it with
certain  sub-advisory  services as described  below in connection with Advisor's
advisory activities on behalf of the Trust;

      WHEREAS,  the advisory  agreement between the Advisor and the Trust, dated
September  29,  2006 (such  agreement  or the most  recent  successor  agreement
between such parties  relating to advisory  services to the Trust is referred to
herein  as  the  "Advisory   Agreement")   contemplates  that  the  Advisor  may
sub-contract  investment  advisory  services  with  respect  to the  Trust  to a
sub-advisor  pursuant to a  sub-advisory  agreement  agreeable  to the Trust and
approved in accordance with the provisions of the 1940 Act; and

      WHEREAS,   this  Agreement  has  been  approved  in  accordance  with  the
provisions  of the 1940 Act,  and the  Sub-Advisor  is willing  to furnish  such
services upon the terms and conditions herein set forth;

      NOW,  THEREFORE,  in  consideration  of the mutual  premises and covenants
herein contained and other good and valuable consideration, the receipt of which
is hereby  acknowledged,  it is  agreed by and  between  the  parties  hereto as
follows:

      1.  Appointment.  The Advisor  hereby  appoints the  Sub-Advisor to act as
sub-advisor  with  respect  to  the  Trust  and  the  Sub-Advisor  accepts  such
appointment  and  agrees  to  render  the  services  herein  set  forth  for the
compensation herein provided.

      2. Services of the  Sub-Advisor.  Subject to the succeeding  provisions of
this section, the oversight and supervision of the Advisor and the direction and
control of the Trust's Board of Trustees,  the Sub-Advisor  will perform certain
of the day-to-day  operations of the Trust, which may include one or more of the
following  services,  at the request of the  Advisor:  (a) acting as  investment
advisor for and managing the investment and  reinvestment of those assets of the
Trust as the Advisor may from time to time request and in  connection  therewith
have complete  discretion in purchasing  and selling such  securities  and other
assets for the Trust and in voting, exercising consents and exercising all other
rights  appertaining to such securities and other assets on behalf of the Trust;
(b) arranging, subject to the provisions of paragraph 3 hereof, for the purchase
and sale of securities and other assets of the Trust;  (c) providing  investment
research and credit analysis concerning the Trust's investments,  (d) assist the
Advisor in determining


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what portion of the Trust's  assets will be invested in cash,  cash  equivalents
and money market instruments,  (e) placing orders for all purchases and sales of
such  investments  made for the Trust, and (f) maintaining the books and records
as are required to support Trust  investment  operations.  At the request of the
Advisor,  the Sub-Advisor will also, subject to the oversight and supervision of
the Advisor and the  direction  and  control of the Trust's  Board of  Trustees,
provide to the  Advisor or the Trust any of the  facilities  and  equipment  and
perform any of the services described in paragraph 3 of the Advisory  Agreement.
In addition,  the  Sub-Advisor  will keep the Trust and the Advisor  informed of
developments  materially  affecting the Trust and shall,  on its own initiative,
furnish  to the Trust from time to time  whatever  information  the  Sub-Advisor
believes  appropriate  for  this  purpose.  The  Sub-Advisor  will  periodically
communicate to the Advisor, at such times as the Advisor may direct, information
concerning the purchase and sale of securities for the Trust, including: (a) the
name of the issuer,  (b) the amount of the purchase or sale, (c) the name of the
broker or dealer,  if any,  through which the purchase or sale is effected,  (d)
the CUSIP number of the  instrument,  if any, and (e) such other  information as
the Advisor may reasonably require for purposes of fulfilling its obligations to
the Trust  under the  Advisory  Agreement.  The  Sub-Advisor  will  provide  the
services  rendered by it under this  Agreement  in  accordance  with the Trust's
investment objectives,  policies and restrictions (as currently in effect and as
they may be amended or supplemented  from time to time) as stated in the Trust's
Prospectus and Statement of Additional  Information  and the  resolutions of the
Trust's Board of Trustees.

      3.  Covenants.  (a) In the performance of its duties under this Agreement,
the Sub-Advisor  shall at all times conform to, and act in accordance  with, any
requirements  imposed by: (i) the  provisions of the 1940 Act and the Investment
Advisers Act of 1940, as amended (the "Advisers  Act") and all applicable  Rules
and Regulations of the Securities and Exchange  Commission (the "SEC"); (ii) any
other  applicable  provision of law; (iii) the provisions of the  Declaration of
Trust and By-Laws of the Trust, as such documents are amended from time to time;
(iv) the  investment  objectives  and  policies of the Trust as set forth in the
Trust's Registration  Statement on Form N-1A and/or the resolutions of the Board
of Trustees; and (v) any policies and determinations of the Board of Trustees of
the Trust and

            (b) In addition, the Sub-Advisor will:

                  (i) place orders  either  directly with the issuer or with any
            broker or dealer. Subject to the other provisions of this paragraph,
            in placing  orders with brokers and dealers,  the  Sub-Advisor  will
            attempt to obtain the best price and the most favorable execution of
            its orders.  In placing orders,  the  Sub-Advisor  will consider the
            experience and skill of the firm's securities traders as well as the
            firm's  financial  responsibility  and  administrative   efficiency.
            Consistent with this obligation,  the Sub-Advisor may select brokers
            on the basis of the research,  statistical and pricing services they
            provide  to the  Trust  and  other  clients  of the  Advisor  or the
            Sub-Advisor.  Information  and research  received  from such brokers
            will be in addition to, and not in lieu of, the services required to
            be performed by the Sub-Advisor hereunder. A commission paid to such
            brokers may be higher than that which another qualified broker would
            have charged for effecting the same  transaction,  provided that the
            Sub-Advisor  determines  in  good  faith  that  such  commission  is
            reasonable  in  terms  either  of the  transaction  or  the  overall
            responsibility  of the Advisor and the  Sub-Advisor to the Trust and
            their other clients and that the total


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<PAGE>

            commissions  paid by the Trust will be reasonable in relation to the
            benefits to the Trust over the  long-term.  Subject to the foregoing
            and the provisions of the 1940 Act, the  Securities  Exchange Act of
            1934,  as  amended,  and other  applicable  provisions  of law,  the
            Advisor may select brokers and dealers with which it or the Trust is
            affiliated;

                  (ii)  maintain  books and records  with respect to the Trust's
            securities  transactions  and will  render  to the  Advisor  and the
            Trust's Board of Trustees such periodic and special  reports as they
            may request;

                  (iii)  maintain  a  policy  and  practice  of  conducting  its
            investment   advisory  services   hereunder   independently  of  the
            commercial   banking   operations  of  its   affiliates.   When  the
            Sub-Advisor  makes  investment  recommendations  for the Trust,  its
            investment   advisory  personnel  will  not  inquire  or  take  into
            consideration whether the issuer of securities proposed for purchase
            or sale for the Trust's  account  are  customers  of the  commercial
            department of its affiliates; and

                  (iv) treat  confidentially  and as proprietary  information of
            the Trust all records and other  information  relative to the Trust,
            and the Trust's prior, current or potential  shareholders,  and will
            not use such  records and  information  for any  purpose  other than
            performance of its  responsibilities  and duties  hereunder,  except
            after prior  notification  to and  approval in writing by the Trust,
            which  approval  shall not be  unreasonably  withheld and may not be
            withheld where the  Sub-Advisor  may be exposed to civil or criminal
            contempt  proceedings  for  failure to  comply,  when  requested  to
            divulge such information by duly constituted authorities, or when so
            requested by the Trust.

      4. Services Not  Exclusive.  Nothing in this  Agreement  shall prevent the
Sub-Advisor or any officer,  employee or other affiliate  thereof from acting as
investment advisor for any other person,  firm or corporation,  or from engaging
in any other  lawful  activity,  and shall not in any way limit or restrict  the
Sub-Advisor or any of its officers,  employees or agents from buying, selling or
trading any  securities  for its or their own  accounts  or for the  accounts of
others  for  whom  it or  they  may  be  acting;  provided,  however,  that  the
Sub-Advisor will undertake no activities which, in its judgment,  will adversely
affect the performance of its obligations under this Agreement.

      5. Books and Records.  In compliance  with the  requirements of Rule 31a-3
under the 1940 Act,  the  Sub-Advisor  hereby  agrees that all records  which it
maintains  for the Trust are the  property  of the Trust and  further  agrees to
surrender  promptly to the Trust any such records upon the Trust's request.  The
Sub-Advisor  further agrees to preserve for the periods prescribed by Rule 31a-2
under the 1940 Act the records required to be maintained by Rule 31a-1 under the
1940 Act (to the  extent  such  books  and  records  are not  maintained  by the
Advisor).

      6. Expenses.  During the term of this Agreement, the Sub-Advisor will bear
all costs  and  expenses  of its  employees  and any  overhead  incurred  by the
Sub-Advisor in connection with its duties hereunder;  provided that the Board of
Trustees  of the Trust  may  approve  reimbursement  to the  Sub-Advisor  of the
pro-rata portion of the salaries, bonuses, health


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<PAGE>

insurance,  retirement  benefits and all similar  employment  costs for the time
spent on Trust operations  (including,  without limitation,  compliance matters)
(other than the  provision  of  investment  advice and  administrative  services
required to be provided  hereunder) of all personnel employed by the Sub-Advisor
who devote  substantial  time to Trust  operations  or the  operations  of other
investment companies advised or sub-advised by the Sub-Advisor.

      7. Compensation.

            (a) The Advisor agrees to pay to the Sub-Advisor and the Sub-Advisor
      agrees to accept as full  compensation  for all  services  rendered by the
      Sub-Advisor  as such,  a monthly fee in arrears at an annual rate equal to
      the amount  set forth in  Schedule  A hereto.  For any period  less than a
      month during which this Agreement is in effect,  the fee shall be prorated
      according to the proportion which such period bears to a full month of 28,
      29, 30 or 31 days, as the case may be.

            (b) For  purposes  of this  Agreement,  the net  assets of the Trust
      shall be calculated  pursuant to the procedures  adopted by resolutions of
      the Trustees of the Trust for  calculating the value of the Trust's assets
      or delegating such calculations to third parties.

      8. Indemnity.

            (a) The Trust may, in the discretion of the Board of Trustees of the
      Trust, indemnify the Sub-Advisor, and each of the Sub-Advisor's directors,
      officers,  employees,  agents,  associates and controlling persons and the
      directors,  partners,  members,  officers,  employees  and agents  thereof
      (including  any  individual  who  serves at the  Sub-Advisor's  request as
      director, officer, partner, member, trustee or the like of another entity)
      (each such  person  being an  "Indemnitee")  against any  liabilities  and
      expenses,   including  amounts  paid  in  satisfaction  of  judgments,  in
      compromise or as fines and penalties, and counsel fees (all as provided in
      accordance  with  applicable  state  law)  reasonably   incurred  by  such
      Indemnitee in connection  with the defense or  disposition  of any action,
      suit or other proceeding,  whether civil or criminal,  before any court or
      administrative  or  investigative  body in which such Indemnitee may be or
      may  have  been  involved  as a party  or  otherwise  or with  which  such
      Indemnitee  may  be or may  have  been  threatened,  while  acting  in any
      capacity  set forth  herein  or  thereafter  by reason of such  Indemnitee
      having acted in any such capacity, except with respect to any matter as to
      which such  Indemnitee  shall have been  adjudicated  not to have acted in
      good faith in the reasonable belief that such  Indemnitee's  action was in
      the  best  interest  of the  Trust  and  furthermore,  in the  case of any
      criminal proceeding, so long as such Indemnitee had no reasonable cause to
      believe  that the conduct was  unlawful;  provided,  however,  that (1) no
      Indemnitee  shall be  indemnified  hereunder  against any liability to the
      Trust or its  shareholders  or any expense of such  Indemnitee  arising by
      reason of (i) willful misfeasance,  (ii) bad faith, (iii) gross negligence
      or (iv) reckless  disregard of the duties  involved in the conduct of such
      Indemnitee's position (the conduct referred to in such clauses (i) through
      (iv) being sometimes referred to herein as "disabling conduct"), (2) as to
      any matter  disposed  of by  settlement  or a  compromise  payment by such
      Indemnitee,  pursuant to a consent decree or otherwise, no indemnification
      either for said payment or for any other expenses shall be provided unless
      there has been a  determination  that such  settlement or compromise is in
      the best interests of the Trust and that such  Indemnitee  appears to have
      acted in


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<PAGE>

      good faith in the reasonable belief that such  Indemnitee's  action was in
      the best  interest of the Trust and did not involve  disabling  conduct by
      such  Indemnitee  and (3)  with  respect  to any  action,  suit  or  other
      proceeding   voluntarily   prosecuted  by  any  Indemnitee  as  plaintiff,
      indemnification shall be mandatory only if the prosecution of such action,
      suit or other  proceeding by such  Indemnitee was authorized by a majority
      of the full Board of Trustees of the Trust.

            (b) The Trust shall make  advance  payments in  connection  with the
      expenses of  defending  any action with  respect to which  indemnification
      might be sought  hereunder if the Trust receives a written  affirmation of
      the Indemnitee's  good faith belief that the standard of conduct necessary
      for  indemnification  has been met and a written  undertaking to reimburse
      the Trust unless it is  subsequently  determined  that such  Indemnitee is
      entitled  to  such  indemnification  and if  the  Trustees  of  the  Trust
      determine   that  the  facts  then  known  to  them  would  not   preclude
      indemnification.  In addition,  at least one of the  following  conditions
      must  be met:  (A) the  Indemnitee  shall  provide  a  security  for  such
      Indemnitee  undertaking,  (B) the Trust  shall be insured  against  losses
      arising by reason of any unlawful  advance,  or (C) a majority of a quorum
      consisting of Trustees of the Trust who are neither  "interested  persons"
      of the Trust (as defined in Section  2(a)(19) of the 1940 Act) nor parties
      to the proceeding  ("Disinterested  Non-Party Trustees") or an independent
      legal counsel in a written opinion, shall determine,  based on a review of
      readily  available facts (as opposed to a full trial-type  inquiry),  that
      there is reason to believe that the  Indemnitee  ultimately  will be found
      entitled to indemnification.

            (c)  All   determinations   with  respect  to  the   standards   for
      indemnification  hereunder  shall be made (1) by a final  decision  on the
      merits by a court or other body  before  whom the  proceeding  was brought
      that such Indemnitee is not liable by reason of disabling conduct,  or (2)
      in the absence of such a decision,  by (i) a majority  vote of a quorum of
      the  Disinterested  Non-Party  Trustees  of the  Trust,  or (ii) if such a
      quorum is not  obtainable  or even, if  obtainable,  if a majority vote of
      such quorum so directs,  independent  legal counsel in a written  opinion.
      All determinations that advance payments in connection with the expense of
      defending any proceeding  shall be authorized  shall be made in accordance
      with the immediately preceding clause (2) above.

      The rights  accruing to any Indemnitee  under these  provisions  shall not
exclude any other right to which such Indemnitee may be lawfully entitled.

      9.  Limitation on Liability.  The  Sub-Advisor  will not be liable for any
error of judgment  or mistake of law or for any loss  suffered by the Advisor or
by the Trust in connection with the performance of this Agreement, except a loss
resulting  from a breach  of  fiduciary  duty with  respect  to the  receipt  of
compensation  for services or a loss  resulting  from willful  misfeasance,  bad
faith or gross  negligence on its part in the  performance of its duties or from
reckless  disregard  by it of its duties under this  Agreement.  As used in this
paragraph  9,  the  term  "Sub-Advisor"  shall  include  any  affiliates  of the
Sub-Advisor  performing services for the Trust contemplated hereby and partners,
directors, officers and employees of the Sub-Advisor and such affiliates.


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<PAGE>

      10. Duration and Termination.  This Agreement shall become effective as of
the date hereof  and,  unless  sooner  terminated  with  respect to the Trust as
provided herein, shall continue in effect for a period of two years. Thereafter,
if not  terminated,  this Agreement shall continue in effect with respect to the
Trust  for  successive  periods  of 12  months,  provided  such  continuance  is
specifically  approved  at least  annually by both (a) the vote of a majority of
the Trust's Board of Trustees or a vote of a majority of the outstanding  voting
securities of the Trust at the time  outstanding and entitled to vote and (b) by
the vote of a majority of the Trustees, who are not parties to this Agreement or
interested  persons (as such term is defined in the 1940 Act) of any such party,
cast in person at a meeting  called for the purpose of voting on such  approval.
Notwithstanding the foregoing,  this Agreement may be terminated by the Trust or
the Advisor at any time,  without the  payment of any  penalty,  upon giving the
Sub-Advisor  60 days' notice  (which  notice may be waived by the  Sub-Advisor),
provided that such  termination by the Trust or the Advisor shall be directed or
approved by the vote of a majority of the Trustees of the Trust in office at the
time or by the vote of the  holders  of a  majority  of the  outstanding  voting
securities  of the Trust  entitled to vote,  or by the  Sub-Advisor  on 60 days'
written  notice (which  notice may be waived by the Trust and the Advisor),  and
will terminate  automatically  upon any  termination  of the Advisory  Agreement
between  the  Trust  and the  Advisor.  This  Agreement  will  also  immediately
terminate in the event of its assignment.  (As used in this Agreement, the terms
"majority  of  the  outstanding  voting  securities,"  "interested  person"  and
"assignment" shall have the same meanings of such terms in the 1940 Act.)

      11.  Notices.  Any notice under this Agreement  shall be in writing to the
other party at such address as the other party may  designate  from time to time
for the receipt of such notice and shall be deemed to be received on the earlier
of the date  actually  received or on the fourth day after the  postmark if such
notice is mailed first class postage prepaid.

      12.  Amendment of this  Agreement.  This  Agreement  may be amended by the
parties only if such amendment is specifically approved by the vote of the Board
of Trustees of the Trust,  including  a majority of those  Trustees  who are not
parties to this Agreement or interested persons of any such party cast in person
at a meeting  called  for the  purpose  of voting on such  approval  and,  where
required  by the 1940 Act,  by a vote of a majority  of the  outstanding  voting
securities of the Trust.

      13.  Miscellaneous.  The  captions  in this  Agreement  are  included  for
convenience  of  reference  only  and in no way  define  or  delimit  any of the
provisions  hereof or otherwise  affect  their  construction  or effect.  If any
provision of this Agreement  shall be held or made invalid by a court  decision,
statute,  rule or  otherwise,  the  remainder  of this  Agreement  shall  not be
affected  thereby.  This  Agreement  shall be binding on, and shall inure to the
benefit of the parties hereto and their respective successors.

      14.  Governing Law. This  Agreement  shall be governed by and construed in
accordance  with the laws of the State of New York for contracts to be performed
entirely  therein without  reference to choice of law principles  thereof and in
accordance  with the  applicable  provisions of the 1940 Act. To the extent that
the applicable laws of the State of New York, or any of the provisions, conflict
with the applicable provisions of the 1940 Act, the latter shall control.


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      15.  Counterparts.  This Agreement may be executed in  counterparts by the
parties hereto, each of which shall constitute an original counterpart,  and all
of which, together, shall constitute one Agreement.



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      IN WITNESS  WHEREOF,  the parties hereto have caused this instrument to be
executed by their duly authorized  officers  designated  below as of the day and
year first above written.


                                    BLACKROCK ADVISORS, LLC


                                    By: ________________________________________
                                         Name:  Donald C. Burke
                                         Title: Managing Director


                                    BLACKROCK INSTITUTIONAL MANAGEMENT
                                    CORPORATION


                                    By: ________________________________________
                                         Name:
                                         Title:

AGREED AND ACCEPTED
as of the date first set forth above

MASTER GOVERNMENT SECURITIES TRUST

By: __________________
    Name:  Donald C. Burke
    Title: Vice President

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                                   Schedule A

                           Sub-Investment Advisory Fee

59% of the monthly advisory fee received by the Advisor from the Trust.