Document:

<PAGE>   1
EXHIBIT 10.31

                              EMPLOYMENT AGREEMENT

THIS AGREEMENT, made and entered into this 1st day of June, 2000, by and between
Martek Biosciences Corporation, a Delaware corporation having its principle
office at 6480 Dobbin Road, Columbia, MD 21045 (the "Company") and George P.
Barker, an individual residing at 10313 Wilde Lake Terrace, Columbia, Maryland
21044 (the "Executive").

WHEREAS, the Company desires to employ the Executive as its Senior Vice
President and General Counsel; and

WHEREAS, the Executive is skilled in areas of law and business development
relevant to the Company's business and desires to enter into employment with the
Company;

NOW THEREFORE, the Company and Executive, in consideration of the premises and
of the respective mutual promises and agreements contained herein, agree with
each other as follows:

        1.      Definitions.

        1.1     Change in Control. Shall be deemed to occur where the Company
has merged or consolidated with or into any other corporation, firm or business
entity or has sold or transferred substantially all of its assets to another
corporation, firm or business entity or has sold fifty percent or more of the
equity investment and voting control in the Company in any twelve month period
other than through a public or private offering of the Company's securities
whereby the primary goal of the new owners of the Company's stock is investment
appreciation or income and not control of the Company.

        1.2     Cause. Shall be defined as those situations where the Executive
has breached this Agreement in any material respect, which breach is not cured
by the Executive or is not capable of being cured within thirty days after
written notice of such breach is delivered to Executive, or where the Executive
has engaged in willful and material failure to perform his duties as an employee
of the Company. Cause shall also include the Executive's

                                   Page 1 of 5

<PAGE>   2

conviction of a criminal offense (other than minor traffic violations) or an act
of moral turpitude, and the Executive's failure to act in the best interests of
the Company or to follow a reasonable direction from the Company's Board of
Directors, which direction is not cured by the Executive or is not capable of
being cured by the Executive within thirty (30) days after written notice is
delivered to the Executive

        1.3     Disability. Disability shall mean the Executive's inability to
perform the essential functions of his job for a continuous period of not less
than one hundred eighty (180) days.

        2.      Term. The term of this Agreement shall be for a period of three
years commencing on the date the Executive begins his employment with the
Company (the "Initial Employment Date") unless earlier terminated as hereinafter
provided. Such Initial Employment Date shall commence on or before June 19,
2000.

        3.      Position and Duties. The Company hereby employs the Executive
and the Executive agrees to work for the Company as its Senior Vice President
and General Counsel during the term of this Agreement, such title however shall
be subject to change at the discretion of the Company's Board of Directors. The
Executive agrees to devote his full time, attention and efforts to the business
and affairs of the Company during the term of this Agreement, and hereby
confirms that he is under no contractual commitments inconsistent with his
obligations as set forth in this Agreement, and that during the term of this
Agreement, he will not render or perform services for any other corporation,
firm, entity or person which are inconsistent with the provisions of this
Agreement. During the term of this Agreement, the Executive agrees to perform
such reasonable employment duties as the Chief Executive Officer or Board of
Directors of the Company may assign to him from time to time.

        4.      Compensation.

        4.1     Base Salary. During the first year of this Agreement, the
Company will pay the Executive a base salary of $167,000.00 per annum, payable
in its normal pay increments in force from time to time (semi-monthly
installments as of the date of this Agreement) for all the services to be
rendered by the Executive under this Agreement. The compensation payab1e to the
Executive during each subsequent year during the term of

                                   Page 2 of 5

<PAGE>   3

this Agreement shall be as mutually agreed to by the Company and Executive prior
to the commencement of each contract year.

        4.2     Incentive Compensation. In addition to the base salary described
in Section 4.1 above, the Executive shall be eligible, in the discretion of the
Company's Board of Directors, to participate in any incentive compensation plans
which may be established by the Board of Directors of the Company from time to
time.

        4.3     Stock Option. The Company shall grant the Executive an option to
purchase fifty thousand (50,000) shares of the Company's common stock, under the
Company's Stock Option Plan, effective on the Executive's Initial Employment
Date. The price per share of stock pursuant to such option will be the closing
price of the Company's common stock as of the common stock's last trading date
prior to the Initial Employment Date. Such stock option shall be subject to the
terms of a separate stock option agreement, but such separate agreement shall
call for the term of the options to be for ten years and for twenty percent
(20%) of the options to vest on the date that is six months after the Initial
Employment Date, an additiona1 twenty percent (20%) of the options to vest on
the date that is one year after the Initial Employment Date, an additional
twenty percent (20%) of the options to vest on the date that is two years after
the Initial Employment Date, an additional twenty percent (20%) of the options
to vest on the date that is three years after the Initial Employment Date and
the balance to vest on the date that is four years after the Initial Employment
Date. To the extent that the terms of the separate stock option agreement are in
conflict, in matters other than the stock option vesting period, with the terms
set forth herein, the terms of the separate stock option agreement shall govern.

        4.4     Participation in Benefit Plans. The Executive shall also be
entitled to participate in all employee benefit plans or programs in place from
time to time (including the Company's Paid Time Off Program which, at the date
of this Agreement, provides twenty-seven (27) paid days off, including vacation
days, sick days, holidays and all other days off, during the first year of the
Executive's employment with the Company) to the extent that his position, title,
tenure, salary, age, health and other qualifications make him eligible to
participate.

                                   Page 3 of 5

<PAGE>   4

        5.0     Termination. Either party may terminate this Agreement by giving
the other party thirty days advance written notice of its intent to terminate.
If the Company terminates this Agreement for reasons other than for Cause as is
defined herein or due to the death or Disability of the Executive, the Company
shall pay the Executive, within thirty (30) days of the effective date of such
termination a "Severance Fee," as additional compensation, equal to twelve (12)
months base salary except that any such Severance Fee shall be reduced by one
twelfth (1/12) for each month that the length of the Agreement, prior to the
effective date of the termination, exceeds twelve (12). Notwithstanding the
above, in the case that a Change of Control has taken effect, and this
Agreement is terminated by the successor organization, such Severance Fee shall
be equal to twelve (12) months base salary. In addition, notwithstanding the
above, the Executive shall be entitled to the payment of a Severance Fee equal
to twelve (12) months base salary if the Executive decides to resign from the
combined or acquired entity due to the occurrence of any of the following: (i)
any downgrading of the Executive's position; (ii) a substantial diminution in
the Executive's duties; (iii) the Executive not being or functioning as the
Senior Vice President and General Counsel of the combined entity; or (iv) the
Executive's decision to refuse a relocation requested by the successor
organization.

        6.0     Assignment. The Executive shall not be permitted to assign this
Agreement. The Company shall have the right to assign this agreement to its
successors or assigns and all covenants and agreements hereunder shall enure to
the benefit of and be enforceable by or against its said successors or assigns.
The terms "successor" and "assign" shall include any corporation, firm or other
business entity with or into which the Company may merge or consolidate, or to
which the Company may sell or transfer all or substantially all of its assets,
or of which fifty percent (50%) or more of the equity investment and of the
voting control is owned, directly or indirectly, by or is in common ownership
with, the Company. After any such assignment by the Company, the Company shall
be discharged from all further liability hereunder and such assignee shall
thereafter be deemed to be the Company for the purposes of all provisions of
this Agreement.

        7.0     Miscellaneous.

                                   Page 4 of 5

<PAGE>   5

        7.1     Governing Law. This Agreement is made under and shall be
governed by and construed in accordance with the laws of the State of Maryland.

        7.2     Entire Agreement, Amendments and No Waiver. This Agreement
constitutes the entire understanding of the parties with respect to the subject
matter hereof and supersedes any and all prior understandings written or oral.
This Agreement may not be changed, modified, or discharged orally, but only by
an instrument in writing signed by the parties The invalidity or
unenforceability of any provisions hereof shall in no way affect the validity or
enforceability of any other provision. If any provision of this Agreement is so
broad as to be unenforceable, such provision shall be interpreted to be only so
broad as is enforceable. No express or implied waiver by either party hereto of
any event of default hereunder shall in any way be, or be construed as, a waiver
of any future or subsequent event of default.

IN WITNESS WHEREOF, the Company has hereunto signed its name by its Chief
Executive Officer, and the Executive has signed his name, all as of the day and
year first above written.

MARTEK BIOSCIENCES CORPORATION

BY:     /s/ HENRY LINSERT, JR.
    -------------------------------------
Name: Henry Linsert, Jr.
Title: Chief Executive Officer

GEORGE P. BARKER

        /s/ GEORGE P. BARKER
------------------------------------------
Executive

                                   Page 5 of 5<PAGE>   1
                                                                     EXHIBIT 4.6

                                FOURTH AMENDMENT

         THIS FOURTH AMENDMENT (this "AMENDMENT") is entered into as of October
30, 2000, among NCI Building Systems, Inc., a Delaware corporation ("BORROWER"),
Bank of America, N.A., successor by merger to NationsBank, N.A., as
Administrative Agent ("AGENT"), UBS AG, Stamford Branch, successor by merger to
Swiss Bank Corporation, as Documentation Agent ("DOCUMENTATION AGENT"), and the
financial institutions named as Lenders therein (collectively, "LENDERS").
Capitalized terms not defined herein have the meaning given such terms in the
Credit Agreement described below.

                                    RECITALS

         A. Borrower, Agent, Documentation Agent, NationsBank Montgomery
Securities LLC, as Syndication Agent, and Lenders executed a Credit Agreement
dated as of March 25, 1998 (as previously modified by the First Amendment dated
May 1, 1998, the Second Amendment dated May 5, 1998, the Waiver, Consent and
Third Amendment dated May 5, 1999, the Waiver and Consent dated November 15,
1999, and the Waiver and Consent dated March 21, 2000, and as amended, restated
or supplemented from time to time, the "CREDIT AGREEMENT").

         B. Borrower has requested certain modifications to SECTIONS 9.9 and
10.4 of the Credit Agreement in return for modifications to SECTIONS 10.2 and
10.3 thereof. Determining Lenders are willing to accept such modifications,
subject to the terms and conditions set forth in this Amendment.

         NOW THEREFORE, in consideration of the premises herein contained and
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the undersigned agree as follows:

         1. Amendments to Credit Agreement. The Credit Agreement is hereby
amended as follows:

                  (a) The first sentence of SECTION 9.9 ("Dividends and
         Distributions") of the Credit Agreement is hereby amended to read in
         its entirety as follows:

                  "Borrower may not declare, make or pay any Distribution, other
                  than (a) Distributions declared, made or paid by Borrower
                  wholly in the form of its capital stock and (b) repurchases by
                  Borrower of up to $50,000,000 of its common stock through a
                  series of open-market transactions or in privately negotiated,
                  off-market transactions."

                  (b) SECTION 10.2 ("Maximum Leverage Ratio") of the Credit
         Agreement is hereby amended so that the ratio for the dates set forth
         below shall be the ratio set forth below opposite such dates:

<TABLE>
<S>                                                  <C>
                           October 31, 2000          3.50 to 1.00
                           Thereafter                3.50 to 1.00
</TABLE>

<PAGE>   2

                  (c) SECTION 10.3 ("Maximum Senior Debt Ratio") of the Credit
         Agreement is hereby amended so that the ratio for the dates set forth
         below shall be the ratio set forth below opposite such dates:

<TABLE>
<S>                                                  <C>
                           October 31, 2000          2.50 to 1.00
                           Thereafter                2.50 to 1.00
</TABLE>

                  (d) SECTION 10.4 ("Minimum Fixed Charge Coverage Ratio") of
         the Credit Agreement is hereby amended by changing the ratio required
         as of October 31, 2000 from 1.35 to 1.00 to 1.30 to 1.00.

                  (e) SCHEDULE 1 to the Credit Agreement is hereby replaced with
         the SCHEDULE 1 attached to this Amendment.

         2. Merger of NationsBank, N.A. and Bank of America, N.A. All references
in the Loan Documents to NationsBank, N.A., and NationsBanc Montgomery
Securities LLC are hereby replaced with references to Bank of America, N.A., and
Banc of America Securities LLC, respectively.

         3. Merger of Swiss Bank Corporation and UBS, AG. All references in the
Loan Documents to Swiss Bank Corporation are hereby replaced with references to
UBS AG, Stamford Branch.

         4. Conditions. This Amendment shall not be effective until (a) it has
been duly executed and delivered by (i) Borrower, (ii) each Guarantor, (iii)
Agent, (iv) Documentation Agent, and (v) at least Determining Lenders, and (b)
Borrower has delivered to Agent for the benefit of each Lender that executes and
delivers this Amendment on or before October 30, 2000, an amendment fee equal to
0.05% of such Lender's Commitment.

         5. Representations and Warranties. Borrower hereby represents and
warrants to Agent and each Lender that: (a) the execution and delivery of this
Amendment has been authorized by all requisite action on its part and will not
violate its organizational documents (and Borrower hereby agrees to furnish
Agent with evidence of such authorization upon request); (b) the representations
and warranties in each Loan Document to which it is a party are true and correct
in all material respects on and as of the date hereof as though made on and as
of the date hereof (except to the extent that (i) such representations and
warranties speak to a specific date, or (ii) the facts on which such
representations and warranties are based have been changed by transactions
contemplated by the Credit Agreement); (c) it is in full compliance with all
Loan Documents to which it is a party; and (d) no default or Potential Default
exists.

         6. Miscellaneous. This Amendment is a Loan Document, and, therefore,
this Amendment is subject to the applicable provisions of SECTION 14 of the
Credit Agreement, all of which applicable provisions are incorporated herein by
reference the same as if set forth herein verbatim. Except as affected by this
Amendment, the Loan Documents are unchanged and continue in full force and
effect. Borrower agrees that all the Loan Documents to which it is a party
remain in full force and effect and continue to evidence its legal, valid, and
binding obligations enforceable in accordance with their terms (as the same are
affected by this Amendment). Borrower hereby releases Agent and each Lender from
any liability for actions or

                                       2
<PAGE>   3

failures to act in connection with the Loan Documents prior to the date hereof.
This Amendment shall be binding upon and inure to the benefit of each of the
undersigned and their respective successors and permitted assigns.

         7. Fees and Expenses. Borrower agrees to pay the reasonable fees and
expenses of counsel to Agent for services rendered in connection with the
preparation, negotiation and execution of this Amendment.

         8. Form. Each agreement, document, instrument or other writing to be
furnished Agent or Lenders under any provision of this Amendment must be in form
and substance satisfactory to Agent and its counsel.

         9. Counterparts. This Amendment may be executed in more than one
counterpart, each of which when so executed shall be deemed to be an original,
but all of which when taken together shall constitute one and the same
instrument.

         10. Final Agreement. THE LOAN DOCUMENTS, AS AMENDED HEREBY, REPRESENT
THE FINAL AGREEMENT AMONG THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF
PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE ARE
NO ORAL, AGREEMENTS AMONG THE PARTIES.

         EXECUTED as of the date first written above.

                                       NCI BUILDING SYSTEMS, INC., as Borrower

                                       By: /s/ Robert J. Medlock
                                          ----------------------------
                                          Robert J. Medlock
                                          Executive Vice President and
                                          Chief Financial Officer

                                       3
<PAGE>   4

                                       BANK OF AMERICA, N.A., as Administrative
                                       Agent and a Lender

                                       By: /s/ Richard L. Nichols, Jr.
                                          --------------------------------------
                                          Richard L. Nichols, Jr.
                                          Managing Director

                                       UBS AG, STAMFORD BRANCH, as Documentation
                                       Agent and a Lender

                                       By: /s/ Wilfred V. Saint
                                          --------------------------------------
                                          Name: Wilfred V. Saint
                                               ---------------------------------
                                          Title: Associate Director
                                                 Banking Product Services US
                                                --------------------------------

                                       By: /s/ Lynne B. Alfarone
                                          --------------------------------------
                                          Name: Lynne B. Alfarone
                                               ---------------------------------
                                          Title: Associate Director
                                                 Banking Product Services US
                                                --------------------------------

                                       THE BANK OF NOVA SCOTIA, as a Lender

                                       By: /s/ A.S. Norsworthy
                                          --------------------------------------
                                          Name: A.S. Norsworthy
                                               ---------------------------------
                                          Title: SR. Team Leader-Loan Operations
                                                --------------------------------

                                       UNION BANK OF CALIFORNIA, N.A., as a
                                       Lender

                                       By: /s/ Hagop V. Jazmadarian
                                          --------------------------------------
                                          Name: Hagop V. Jazmadarian
                                               ---------------------------------
                                          Title: Vice President
                                                --------------------------------

                                       4
<PAGE>   5

                                       IMPERIAL BANK, as a Lender

                                       By: /s/ Ray Vadalma
                                          --------------------------------------
                                          Name: Ray Vadalma
                                               ---------------------------------
                                          Title: Senior Managing Director
                                                --------------------------------

                                       FIRST UNION NATIONAL BANK, as a Lender

                                       By: /s/ David C. Hauglid
                                          --------------------------------------
                                          Name: David C. Hauglid
                                               ---------------------------------
                                          Title: Vice President
                                                --------------------------------

                                       CREDIT INDUSTRIEL ET COMMERICAL, as a
                                       Lender

                                       By: /s/ Anthony Rock
                                          --------------------------------------
                                          Name: Anthony Rock
                                               ---------------------------------
                                          Title: Vice President
                                                --------------------------------

                                       By: /s/ Brian O'Leary
                                          --------------------------------------
                                          Name: Brian O'Leary
                                               ---------------------------------
                                          Title: Vice President
                                                --------------------------------

                                       COMERICA BANK, as a Lender

                                       By: /s/ T. Brancroft Mattei
                                          --------------------------------------
                                          Name: T. Brancroft Mattei
                                               ---------------------------------
                                          Title: Account Officer
                                                --------------------------------

                                       5
<PAGE>   6

                                       CREDIT LYONNAIS NEW YORK BRANCH, as a
                                       Lender

                                       By: /s/ Attla Koc
                                          --------------------------------------
                                          Name: Attla Koc
                                               ---------------------------------
                                          Title: Sr. V.P.
                                                --------------------------------

                                       BANK AUSTRIA CREDITANSTALT CORPORATE
                                       FINANCE, INC., as a Lender

                                       By: /s/ John Taylor
                                          --------------------------------------
                                          Name: John Taylor
                                               ---------------------------------
                                          Title: Vice President
                                                --------------------------------

                                       By: /s/ Scott Kray
                                          --------------------------------------
                                          Name: Scott Kray
                                               ---------------------------------
                                          Title: Senior Vice President
                                                --------------------------------

                                       GENERAL ELECTRIC CAPITAL CORPORATION, as
                                       a Lender

                                       By: /s/ Gregory Hong
                                          --------------------------------------
                                          Name: Gregory Hong
                                               ---------------------------------
                                          Title: Duly Authorized Signatory
                                                --------------------------------

                                       WACHOVIA BANK, N.A., as a Lender

                                       By:
                                          --------------------------------------
                                          Name:
                                               ---------------------------------
                                          Title:
                                                --------------------------------

                                       6
<PAGE>   7

                                       CIBC, INC., as a Lender

                                       By: /s/ Stephanie E. DeVane
                                          --------------------------------------
                                          Name: Stephanie E. DeVane
                                               ---------------------------------
                                          Title: Executive Director
                                                 CIBC World Markets Corp., as
                                                 Agent
                                                --------------------------------

                                       CREDIT AGRICOLE INDOSUEZ, as a Lender

                                       By: /s/ Patrick Cocquerel
                                          --------------------------------------
                                          Name: Patrick Cocquerel
                                               ---------------------------------
                                          Title: FVP, Managing Director
                                                --------------------------------

                                       By: /s/ Michael R. Quiray
                                          --------------------------------------
                                          Name: Michael R. Quiray
                                               ---------------------------------
                                          Title: VP, SR Manager
                                                --------------------------------

                                       THE FUJI BANK, LIMITED, as a Lender

                                       By: /s/ Nohucki Koike
                                          --------------------------------------
                                          Name: Nohuoki Koike
                                               ---------------------------------
                                          Title: Vice President & Senior Team
                                                 Leader
                                                --------------------------------

                                       THE INDUSTRIAL BANK OF JAPAN, LIMITED, as
                                       a Lender

                                       By: /s/ J. Kenneth Biegen
                                          --------------------------------------
                                          Name: J. Kenneth Biegen
                                               ---------------------------------
                                          Title: Senior Vice President
                                                --------------------------------

                                       7
<PAGE>   8

                                       THE SUMITOMO BANK, as a Lender

                                       By: /s/ Suresh S. Tata
                                          --------------------------------------
                                          Name: Suresh S. Tata
                                               ---------------------------------
                                          Title: Senior Vice President
                                                --------------------------------

                                       SOUTHWEST BANK OF TEXAS, N.A., as a
                                       Lender

                                       By: /s/ Gary Tolbert
                                          --------------------------------------
                                          Name: Gary Tolbert
                                               ---------------------------------
                                          Title: Sr. V.P.
                                                --------------------------------

                                       SUNTRUST BANK, as a Lender

                                       By: /s/ Donald L. Gaudette, Jr.
                                          --------------------------------------
                                          Name: Donald L. Gaudette, Jr.
                                               ---------------------------------
                                          Title: Director
                                                --------------------------------

                                       NATIONAL CITY BANK, as a Lender

                                       By: /s/ Scott Brewer
                                          --------------------------------------
                                          Name: Scott Brewer
                                               ---------------------------------
                                          Title: VP
                                                --------------------------------

                                       TEXTRON FINANCIAL CORPORATION, as a
                                       Lender

                                       By: /s/ Stuart Schulman
                                          --------------------------------------
                                          Name: Stuart Schulman
                                               ---------------------------------
                                          Title: Managing Director
                                                --------------------------------

                                       8
<PAGE>   9

                               GUARANTORS' CONSENT

Each of the undersigned:

(a)      consents and agrees to this Amendment and agrees that the Loan
         Documents to which it is a party shall remain in full force and effect
         and shall continue to be the legal, valid, and binding obligations of
         the undersigned enforceable in accordance with their terms:

(b)      represents and warrants to each Lender that: (a) the execution and
         delivery of this Amendment have been authorized by all requisite action
         on its part and will not violate its organizational documents (and
         agrees to furnish Agent with evidence of such authorization upon
         request); (b) the representations and warranties in each Loan Document
         to which it is a party are true and correct in all material respects on
         and as of the date hereof as though made on and as of the date hereof
         (except to the extent that (i) such representations and warranties
         speak to a specific date, or (ii) the facts on which such
         representations and warranties are based have been changed by
         transactions contemplated by the Credit Agreement); and (c) it is in
         full compliance with all Loan Documents to which it is a party; and (d)
         no Default or Potential Default exists; and

(c)      releases Agent, Documentation Agent and each Lender from any liability
         for actions or failures to act in connection with the Loan Documents
         prior to the date hereof.

                                       NCI HOLDING CORP.
                                       NCI OPERATING CORP.
                                       METAL COATERS OF CALIFORNIA, INC.
                                       DOUBLECOTE, L.L.C.

                                       By: /s/ Robert J. Medlock
                                          --------------------------------------
                                          Robert J. Medlock
                                          Executive Vice President and
                                          Chief Financial Officer

                                       A & S BUILDING SYSTEMS, L.P.
                                       NCI BUILDING SYSTEMS, L.P.
                                       METAL BUILDING COMPONENTS, L.P.
                                       METAL COATERS OPERATING, L.P.

                                       By: NCI OPERATING CORP.,
                                           as General Partner

                                       By: /s/ Robert J. Medlock
                                          --------------------------------------
                                          Robert J. Medlock
                                          Executive Vice President and
                                          Chief Financial Officer

                                       9
<PAGE>   10

                                   SCHEDULE 1
                            (As of October 30, 2000)

Borrower and Guarantors

NCI Building Systems, Inc.
10943 North Sam Houston Parkway West
Houston, Texas 77064
Attn: Robert J. Medlock
      Chief Financial Officer
FAX: (281) 477-9675

Agent

Bank of America, N.A.
NCI-007-17-11
100 North Tryon Street, 17th Floor
Charlotte, North Carolina 28255-0001
Attn: Richard L. Nichols, Jr.
      Managing Director
FAX: (704) 386-3271

Copy to:

Porter & Hedges, L.L.P.
700 Louisiana, 35th Floor
Houston, Texas 77002
Attn: F. Walter Bistline, Jr.
FAX: (713) 226-0281

                                       10

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00019-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00019-of-00352.parquet"}]]