Exhibit 10.45.1


                       FIRST AMENDMENT TO PROMISSORY NOTE


     THIS FIRST  AMENDMENT TO PROMISSORY NOTE (the  "Amendment"),  is made as of
June 30, 2003 by and between MISSION WEST  PROPERTIES,  L.P., a Delaware limited
partnership  ("Borrower"),  and CITICORP  U.S.A.,  INC., a Delaware  corporation
("Lender") as follows:

                                    RECITALS

     A. In  connection  with a loan (the  "Loan"),  made by Lender to  Borrower,
Borrower  executed  that  certain  Promissory  Note,  dated as of April 8, 2003,
payable  to  Lender,  in the  amount of  $80,000,000  (the  "Note").  Borrower's
obligations  under the Note are secured,  among other security,  by that certain
Deed of Trust,  Assignment of Leases and Rents,  Security  Agreement and Fixture
Filing (the "Deed of Trust"), dated as of April 8, 2003 executed by Borrower, as
trustor,  in favor of Lender,  as  Beneficiary,  and  recorded  in the  official
records of Santa Clara County,  California  on April 8, 2003 as document  number
16947181, and guaranteed by that certain Guaranty (the "Guaranty"),  dated as of
April 8, 2003 executed by Carl E. Berg, an individual,  Mission West Properties,
L.P. I, a Delaware  limited  partnership,  Mission West  Properties,  L.P. II, a
Delaware limited  partnership and Mission West Properties,  L.P. III, a Delaware
limited  partnership  (collectively,  "Guarantor").  Borrower and Guarantor have
also  executed  and  delivered to Lender that  certain  Environmental  Indemnity
Agreement  (the  "Environmental  Indemnity")  dated April 8, 2003. The Note, the
Deed  of  Trust,  the  Guaranty,  the  Environmental  Indemnity  and  the  other
documents,  instruments  and  agreements  evidencing  or  securing  the Loan are
hereinafter referred to collectively as the "Loan Documents".

     B. Borrower has  requested  and Lender has agreed,  subject to the terms of
this Amendment,  to modify the terms of the Note to extend the Maturity Date (as
defined in the Note).

NOW,  THEREFORE,  in  consideration of the foregoing and for other good valuable
consideration,  the receipt and  sufficiency  of which are hereby  acknowledged,
Lender and Borrower hereby agree as follows:

                                    AGREEMENT

     1. DEFINITIONS.  Except as modified herein, terms defined in the Note shall
have the same meaning when used in this Amendment.

     2.  MODIFICATION  TO NOTE.  On and  after the date of this  Amendment,  the
definition of "Maturity  Date" set forth in Paragraph  1(a) of the Note shall be
amended and modified to mean April 6, 2004.

     3.  REAFFIRMATION  OF NOTE AND OTHER  LOAN  DOCUMENTS.  Except as  modified
pursuant to Paragraph 2 hereof, the Note is unmodified and remains in full force
and effect.

     4.  CONDITIONS  PRECEDENT.  Before this Amendment  becomes  effective,  the
following  conditions  shall be satisfied in a manner  satisfactory to Lender in
the  exercise of its  reasonable  judgment:  (a) no Event of Default  shall have
occurred  and be  continuing  and no other  event  shall  have  occurred  and be
continuing  if the event,  with notice or the passage of time or both,  would be
such an Event of  Default;  (b) if  requested  by  Lender,  Borrower  shall have
provided such title  endorsements as Lender may request to insure the continuing
first-lien  priority of the Deed of Trust;  (c) no material adverse change shall
have  occurred,  or be  reasonably  likely  to  occur,  in  the  Property  or in
Borrower's or any  Guarantor's  business  conditions  (financial or  otherwise),
operations,  properties  or  prospects,  or ability to repay the Loan;  (d) each
Guarantor  shall have executed the  reaffirmation  of its  obligations  attached
hereto;  and (f) Lender  shall have  received an  extension  fee in  immediately
available funds in the amount of Fifty Thousand and 00/100 Dollars ($50,000.00).


     5. BORROWER'S REPRESENTATIONS AND WARRANTIES. Borrower hereby reaffirms all
of the  representations  and  warranties  set  forth in the Loan  Documents  and
further  represents  and warrants  that: (a) the recitals set forth above in the
Recitals  are true,  accurate  and  correct;  (b) Borrower is the sole legal and
beneficial  owner of the  Property;  (c) the Deed of Trust  constitutes a valid,
first priority lien  encumbering the Property and there are no other  mortgages,
deeds of trust or other such  liens  encumbering  the  Property  or any  portion
thereof; (d) this Amendment  constitutes the legal, valid and binding obligation
of  Borrower  enforceable  in  accordance  with its  terms;  (e) the  execution,
delivery and  performance  of this  Amendment  are within  Borrower's  power and
authority and have been duly authorized by all requisite partnership action, and
are not in  contravention  of any law, or of Borrower's  certificate  of limited
partnership or partnership agreement; (f) there exists no Event of Default under
the Note or any other  Loan  Document;  and (g) there  are no  offsets,  claims,
counterclaims, cross-claims or defenses with respect to the Loan.

     6. GOVERNING  LAW;  SEVERABILITY.  This Amendment  shall be governed by and
construed  under the internal  laws (as opposed to the laws of conflicts) of the
State of California. In the event that any provision or clause of this Amendment
is  construed

<PAGE>

by a court of competent jurisdiction to be void, invalid or unenforceable,  such
construction  shall not affect other  provisions of this Amendment  which can be
given effect without the void, invalid or unenforceable  provision,  and to this
end the provisions of this Amendment are declared to be severable.

     7. CAPTIONS.  Titles and headings  appearing in this Amendment are intended
solely  for  means  of  reference  and are not  intended  to  modify  any of the
provisions of this Amendment.

     8.  ENTIRE  AGREEMENT.  This  Amendment  constitutes  the entire  agreement
between Borrower and Lender with respect to the subject matter of this Amendment
and may not be modified or amended in any manner except in writing  executed and
delivered by Borrower and Lender.

     9. COUNTERPARTS.  This Amendment may be executed in multiple  counterparts,
each of which, taken together, shall constitute one and the same agreement.

         IN WITNESS WHEREOF, this Amendment has been duly executed as of the
date set forth above.




                                         BORROWER

                                         MISSION WEST PROPERTIES, L.P.,
                                         a Delaware limited partnership

                                         By:      Mission West Properties, Inc.,
                                                  a Maryland corporation
                                                  its general partner

                                                  By:   /s/ Carl E. Berg
                                                  Name:  Carl E. Berg
                                                  Title:  CEO


                                         LENDER

                                         CITICORP U.S.A., INC.,
                                         a Delaware corporation

                                         By:   /s/ David Taylor
                                         Name:   David Taylor
                                         Title:  Vice President



<PAGE>


                           REAFFIRMATION OF GUARANTORS

Guarantor  hereby (a)  represents and warrants to Lender that, if Guarantor is a
partnership, the execution,  delivery, and performance of this Reaffirmation are
within  its  partnership  powers,  have been duly  authorized  by all  necessary
partnership  action;  (b)  represents and warrants to Lender that the execution,
delivery, and performance of this Reaffirmation shall not constitute a breach of
any other  document,  instrument or agreement to which it is a party or by which
its  property is bound;  (c)  consents to the  amendment  of the Loan  Documents
pursuant to and on the terms stated in the Amendment,  including but not limited
to the extension of the Maturity  Date to April 6, 2004;  (d)  acknowledges  and
reaffirms its obligations owing to Lender under the Guaranty,  the Environmental
Indemnity and any other Loan Documents to which it is a party; and (e) ratifies,
affirms, reaffirms, acknowledges, confirms agrees that each of the Guaranty, the
Environmental  Indemnity and any other Loan  Documents to which it is a party is
and shall remain in full force and effect and represents a valid and enforceable
obligation of the Guarantor.


                                         GUARANTOR:

                                         /s/ Carl E. Berg
                                         Carl E. Berg, an individual

                                         MISSION WEST PROPERTIES, L.P. I,
                                         a Delaware limited partnership

                                         By:      Mission West Properties, Inc.,
                                                  a Maryland corporation
                                                  its general partner

                                                  By:   /s/ Carl E. Berg
                                                  Name:  Carl E. Berg
                                                  Title:  CEO


                                         MISSION WEST PROPERTIES, L.P. II,
                                         a Delaware limited partnership

                                         By:      Mission West Properties, Inc.,
                                                  a Maryland corporation
                                                  its general partner

                                                  By:   /s/ Carl E. Berg
                                                  Name:  Carl E. Berg
                                                  Title:  CEO

                                         MISSION WEST PROPERTIES, L.P. III,
                                         a Delaware limited partnership

                                         By:      Mission West Properties, Inc.,
                                         a Maryland corporation
                                         its general partner

                                         By:   /s/ Carl E. Berg
                                         Name:   Carl E. Berg
                                         Title:   CEO