Document:

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                                                                   EXHIBIT 10.37

                       THIRD AMENDMENT OF LEASE AGREEMENT

        THIS THIRD AMENDMENT OF LEASE, made as of this 15th day of September,
1994, between REGIONAL INDUSTRIAL DEVELOPMENT CORPORATION OF SOUTHWESTERN
PENNSYLVANIA, a Pennsylvania nonprofit corporation having its principal office
in the City of Pittsburgh, Allegheny County, Pennsylvania (hereinafter called
the "Lessor"), and TOLLGRADE COMMUNICATIONS, INC., a Pennsylvania corporation
(hereinafter called the "Lessee").

        WHEREAS, the Lessor and Lessee previously made and entered into that
certain Lease Agreement, dated as of August 5, 1993, which was amended by the
First Amendment of Lease dated March 15, 1994 and Second Amendment of Lease
dated July 1, 1994 (hereinafter collectively called the "Lease"), whereby Lessor
leased to Lessee, upon terms and conditions set forth therein, certain space
(the "Premises") located in the Harmar Industrial Manor (hereinafter called the
"Complex") located in Harmar Township, Allegheny County, Pennsylvania; and

        WHEREAS, the Lessor and the Lessee desire to amend the Lease upon the
terms and conditions set forth herein such that Lessee shall lease from Lessor
additional space in the Complex (hereinafter called the "Amendment of Lease").

        NOW THEREFORE, for good and valuable consideration, the receipt of which
hereby acknowledged, and intending to be legally bound hereby, Lessor and Lessee
covenant and agree as follows:

        1.      Incorporation by Reference/Capitalized Terms. The Lease is
hereby incorporated into this Amendment of Lease by reference

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Third Amendment of Lease that is not otherwise defined herein shall have the
respective meaning ascribed to it in the Lease. In the event of a conflict
between the terms of this Amendment of Lease and the Lease, the terms of this
Amendment of Lease shall govern.

        2.      Premises. The Lessor does hereby lease and demise unto the
Lessee Unit D-4, containing approximately 800 rentable square feet, in the
Complex (hereinafter called the "Fourth Additional Space"). The Fourth
Additional Space is more particularly described on Exhibit "A", which is
attached hereto and made part hereof. The Fourth Additional Space shall be
deemed hereafter to be part of the Premises, as that term is defined in the
Lease.

        3.      Commencement Date. The term of the Amendment of Lease shall
commence (hereinafter called the "Commencement Date") and possession shall be
given as of 12:01 AM on September 15, 1994. The Term of this Amendment of Lease
shall expire on the Expiration Date as set forth in the Lease unless the Term is
extended as provided in the Lease.

        4.      Base Rent. Lessee shall pay to Lessor at its office in
Pittsburgh, as monthly rental for the Fourth Additional Space, payable in
advance and without demand on the first business day of each successive calendar
month during the term of the lease, together with any escalations provided in
the Lease, the following amounts:

        For the period September 15, 1994 through September 30, 1994, the sum of
ONE HUNDRED FIFTY AND 00/100 DOLLARS ($150.00).

        For the period October 1, 1994 through December 31, 1998, the sum of
THREE HUNDRED AND 00/100 DOLLARS ($300.00).

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        If the Commencement Date is on a day other than the first business day
of a calendar month, Lessee shall pay to Lessor on the date that the first
complete payment of Fourth Additional Space Base Rent is due, a pro rata portion
to be based on the number of days the Fourth Additional Space is occupied in
such partial month after the Commencement Date.

        5.      Full Force and Effect. Except as extended and amended hereby,
all other terms and conditions of the Lease shall remain unchanged and in full
force and effect and applicable to this Amendment of Lease.

        IN WITNESS WHEREOF, the parties hereto have caused this Third Amendment
of Lease to be executed the day and year first above written.

ATTEST:                                    REGIONAL INDUSTRIAL DEVELOPMENT
                                           CORPORATION OF SOUTHWESTERN
                                           PENNSYLVANIA

BY /s/ Colleen B. Poremski                 By /s/ Frank Brooks Robinson
   -----------------------------             -----------------------------------
        Secretary                                   President

(Corporate Seal)

ATTEST:                                    TOLLGRADE COMMUNICATIONS, INC.

By /s/ Renee Sanders                       By /s/ Christian L. Allison
   -----------------------------             -----------------------------------
        V.P. Acct./Purchasing                       President

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<PAGE>

                                    EXHIBIT A

        [FLOOR PLAN OF FOURTH ADDITIONAL SPACE]<PAGE>

                                                                   EXHIBIT 10.38

                       FOURTH AMENDMENT OF LEASE AGREEMENT

        THIS FOURTH AMENDMENT OF LEASE, made as of this 15th day of September,
1994, between REGIONAL INDUSTRIAL DEVELOPMENT CORPORATION OF SOUTHWESTERN
PENNSYLVANIA, a Pennsylvania nonprofit corporation having its principal office
in the City of Pittsburgh, Allegheny County, Pennsylvania (hereinafter called
the "Lessor"), and TOLLGRADE COMMUNICATIONS, INC., a Pennsylvania corporation
(hereinafter called the "Lessee").

        WHEREAS, the Lessor and Lessee previously made and entered into that
certain Lease Agreement, dated as of August 5, 1993, which was amended by the
First Amendment of Lease dated March 15, 1994, Second Amendment of Lease dated
July 1, 1994 and Third Amendment of Lease dated September 15, 1994 (hereinafter
collectively called the "Lease"), whereby Lessor leased to Lessee, upon terms
and conditions set forth therein, certain space (the "Premises") located in the
Harmar Industrial Manor (hereinafter called the "Complex") located in Harmar
Township, Allegheny County, Pennsylvania; and

        WHEREAS, the Lessor and the Lessee desire to amend the Lease upon the
terms and conditions set forth herein such that Lessee shall lease from Lessor
additional space in the Complex (hereinafter called the "Amendment of Lease").

        NOW THEREFORE, for good and valuable consideration, the receipt of which
hereby acknowledged, and intending to be legally bound hereby, Lessor and Lessee
covenant and agree as follows:

        1.      Incorporation by Reference/Capitalized Terms. The Lease is
hereby incorporated into this Amendment of Lease by reference

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and made an integral part hereof. Any capitalized term used in the Fourth
Amendment of Lease that is not otherwise defined herein shall have the
respective meaning ascribed to it in the Lease. In the event of a conflict
between the terms of this Amendment of Lease and the Lease, the terms of this
Amendment of Lease shall govern.

        2.      Premises. The Lessor does hereby lease and demise unto the
Lessee Units D-2, D-3 and D-5, containing approximately 4,440 rentable square
feet, in the Complex (hereinafter called the "Fifth Additional Space"). The
Fifth Additional Space is more particularly described on Exhibit "A", which is
attached hereto and made part hereof. The Fifth Additional Space shall be deemed
hereafter to be part of the Premises, as that term is defined in the Lease.

        3.      Commencement Date. The term of the Amendment of Lease shall
commence (hereinafter called the "Commencement Date") and possession shall be
given as of 12:01 AM on October 1, 1994. The Term of this Amendment of Lease
shall expire on the Expiration Date as set forth in the Lease unless the Term is
extended as provided in the Lease.

        4.      Base Rent. Lessee shall pay to Lessor at its office in
Pittsburgh, as monthly rental for the Fifth Additional Space, payable in advance
and without demand on the first business day of each successive calendar month
during the term of the lease, together with any escalations provided in the
Lease, following amount of ONE THOUSAND SIX HUNDRED SIXTY-FIVE AND 00/100
DOLLARS ($1665.00).

        If the Commencement Date is on a day other than the first

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business day of a calendar month, Lessee shall pay to Lessor on the date that
the first complete payment of Fifth Additional Space Base Rent is due, a pro
rata portion to be based on the number of days the Fifth Additional Space is
occupied in such partial month after the Commencement Date.

        5.      Full Force and Effect. Except as extended and amended hereby,
all other terms and conditions of the Lease shall remain unchanged and in full
force and effect and applicable to this Amendment of Lease.

        IN WITNESS WHEREOF, the parties hereto have caused this Fourth Amendment
of Lease to be executed the day and year first above written.

ATTEST:                                    REGIONAL INDUSTRIAL DEVELOPMENT
                                           CORPORATION OF SOUTHWESTERN
                                           PENNSYLVANIA

By /s/ Colleen B. Poremski                 By /s/ Frank Brooks Robinson
  ------------------------------             --------------------------
         Secretary                                 President

(Corporate Seal)

ATTEST:                                    TOLLGRADE COMMUNICATIONS, INC.

By /s/ Renee Sanders                       By /s/ Christian L. Allison
  ------------------------------             -------------------------
         V.P. Acct/Purchasing                      President

                                      -3-
<PAGE>

                                    EXHIBIT A

[FLOOR PLAN OF FIFTH ADDITIONAL SPACE]<PAGE>

                                                                  EXHIBIT 10.39

                       FIFTH AMENDMENT OF LEASE AGREEMENT

         THIS FIFTH AMENDMENT OF LEASE AGREEMENT (this "Amendment"), is made and
entered into this 6th day of March, 1995, between REGIONAL INDUSTRIAL
DEVELOPMENT CORPORATION OF SOUTHWESTERN PENNSYLVANIA, ("Lessor"), and TOLLGRADE
COMMUNICATIONS, INC. ("Lessee").

                                   WITNESSETH:

         WHEREAS, Lessor and Lessee entered into a Lease Agreement for certain
space in Harmar Industrial Manor in Harmar Township, Allegheny County,
Pennsylvania (herein called the "Complex"), dated August 5, 1993, which Lease
was amended by the First Amendment of Lease Agreement dated March 15, 1994,
Second Amendment of Lease Agreement dated as of July 1, 1994, Third Amendment of
Lease Agreement dated as of September 15, 1994 and Fourth Amendment of Lease
Agreement made as of September 15, 1994 (hereinafter collectively called the
"Lease"); and

         WHEREAS, Lessee desires to utilize additional space in the Complex in
connection with its use of the Premises subject to the Lease; and

         WHEREAS, Lessor is agreeable to such use of the additional space by the
Lessee as long as such use is in accordance with the terms of this Amendment,
including certain clarifications to the Lease.

         NOW THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, and intending to be legally bound
hereby, Lessor and Lessee agree as follows:

         1. Recitals. The foregoing recitals are incorporated herein by
reference.

         2. Incorporation of Lease / Capitalized Terms. This Amendment is hereby
incorporated into the Lease by reference and made an integral part thereof.
Capitalized terms used herein but not defined herein shall have the meaning
given in the Lease. In the event of a conflict between the terms of this
Amendment and the Lease, the terms of this Amendment shall govern.

         3. IDL. All referenced in Section 1C of the Lease to "IDC" are deleted
in their entirety and replaced by the name "IDL".

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         4. Partial IDL Space. The following sentence is inserted after the
second sentence in Section 1C of the Lease:

         "Lessee shall have the right to lease all such IDL space or a portion
         of such IDL space, provided, however, if Lessee desires to lease only a
         portion of the IDL space, Lessee agrees to lease at least 4,000 square
         feet of such IDL space at a time. Lessee shall, at its own cost and
         expense, install any partitions in the IDL space which are required on
         account of Lessee leasing less than all of the IDL space."

         5. Utilities. The following sentence is inserted at the end of Section
1C of the Lease:

         "Any time that Lessee takes additional space in the Complex, Lessee
         shall, at its sole cost and expense, be responsible to cause the
         utilities serving such space to be separately metered so that Lessee
         shall pay all utility charges for the Premises, including any new space
         so leased, directly. Lessee shall, at its sole cost and expense,
         additionally separate the gas space heater in Unit D-6 from Lessor's
         main system. All work done by Lessee pursuant to this Section 1C shall
         be subject to all applicable laws, rules, regulations and orders and
         subject to prior review of plans submitted by Lessee to Lessor and
         consent of Lessor, which shall not be unreasonably withheld."

         6. New Space. The Lessor does hereby lease and demise unto the Lessee
Suite 104 containing One Thousand One Hundred Fifty (1,150) rentable square feet
(the "Space"). The Space is more particularly described on Exhibit A attached
hereto and incorporated herein. The Space shall be deemed to be par the Premises
under the Lease.

         7. Mechanical Equipment. Lessee shall, at its so and expense, be
responsible to cause the HVAC duct work serving the Space to be separated from
the main system at the Complex and connected to the HVAC system serving the
Lessee's Premises. All work done by Lessee hereunder shall be subject to all
applicable laws, rules, regulations and orders and subject to prior review of
plans submitted by Lessee to Lessor and consent of Lessor, which shall not be
unreasonably withheld. Nothing herein shall be interpreted to affect the
Lessor's obligations pursuant to Section 33 of the Lease.

         8. Commencement Date. The term of this Amendment shall commence and
possession shall be given as of 12:01 AM on May 1, 1995 (the "Space Date"). The

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term of this Amendment shall expire on the Expiration Date as set forth in the
Lease, unless sooner terminated or extended in accordance with the terms of the
Lease.

         9. Base Rent. Lessee shall pay to Lessor at Lessor's office in
Pittsburgh, the sum of Seven Hundred Ninety Dollars and Sixty Three Cents
($790.63) as monthly rental for the Space (being the monthly calculation of
rental based on $8.25/square foot/year), payable in advance and without demand
on or before the first business day of each successive calendar month of the
Term of the Lease and any renewal thereof, which rental shall be deemed to be
additional Base Rental.

         10. Simtronics Space. In connection with the Lessee's option to lease
the space in the Complex currently occupied by Simtronics, Lessor and Lessee
agree that the rental payable by Lessee upon exercise of such option shall,
effective for a period of six (6) months following Simtronics' surrender of
possession of such space, be $8.25 per square foot per year. Provided that
Lessor has made the Simtronics space available by July 1, 1995, Lessee agrees
that it shall exercise its option and lease such space at the rate set forth
herein, by means of a separate amendment to the Lease containing such other
terms as are mutually acceptable to the parties.

         11. Governing Law. This Amendment shall be governed by the laws of the
Commonwealth of Pennsylvania.

         12. Lease. Except as amended hereby, the Lease shall remain in full
force and effect and shall be binding upon the parties hereto and their
respective successors and assigns.

                                      -3-

<PAGE>

         IN WITNESS WHEREOF, the parties hereto have caused this Amendment of
Lease to be duly executed the day and year first above written.

                                            Lessor:

                                            REGIONAL INDUSTRIAL DEVELOPMENT
                                            CORPORATION OF SOUTHWESTERN
                                            PENNSYLVANIA

                                            By:  /s/ Frank Brooks Robinson
                                               -------------------------------
                                            Name:
                                                 -----------------------------
                                            Title:  President
                                                  ----------------------------

                                            Lessee:

                                            TOLLGRADE COMMUNICATIONS, INC.

                                            By:  /s/ R. Craig Allison
                                               -------------------------------
                                            Name:  R. CRAIG ALLISON
                                                 -----------------------------
                                            Title:  COB/CEO
                                                  ----------------------------

                                      -4-
<PAGE>

                                    EXHIBIT A

[FLOOR PLAN OF SPACE]

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