FIRST AMENDMENT TO CREDIT AGREEMENT

Exhibit 10.121

FIRST AMENDMENT TO CREDIT AGREEMENT

THIS FIRST AMENDMENT TO CREDIT AGREEMENT (this “First Amendment”), dated as of October 18, 2005, is among NEXSTAR BROADCASTING, INC., a Delaware corporation (the “Borrower”), NEXSTAR BROADCASTING GROUP, INC., a Delaware corporation (the “Ultimate Parent”), NEXSTAR FINANCE HOLDINGS, INC., the several Lenders (as such term is defined in the hereinafter described Credit Agreement) parties to this First Amendment, and BANK OF AMERICA, N.A., as Administrative Agent for the Lenders (in such capacity, the “Administrative Agent”).

R E C I T A L S:

A. The Borrower, the Ultimate Parent, certain of its Subsidiaries from time to time party to the Credit Agreement, the Administrative Agent, UBS Securities LLC and Merrill Lynch, Pierce, Fenner & Smith Incorporated, as the Co-Syndication Agents, and the several Lenders parties thereto entered into that certain Fourth Amended and Restated Credit Agreement dated as of April 1, 2005 (as the same may be amended, restated or modified from time to time, the “Credit Agreement”). Capitalized terms used and not otherwise defined herein shall have the meanings ascribed to them in the Credit Agreement.

B. The Borrower has advised the Administrative Agent that it desires to amend the Credit Agreement to modify certain financial covenants under the Credit Agreement.

C. The Majority Revolver Lenders and the Majority Lenders parties to this First Amendment are willing to agree to the above-described amendment, subject to the performance and observance in full of each of the covenants, terms and conditions, and in reliance upon all of the representations and warranties of the Borrower and the Parent Guarantors, set forth herein.

NOW, THEREFORE, in consideration of the premises and the covenants, terms and conditions, and in reliance upon the representations and warranties, in each case contained herein, the parties hereto agree hereby as follows:

ARTICLE I

Amendments

Section 1.01 Amendment to Section 7.09(a). Effective as of the Effective Date (as defined in Section 4.05), Section 7.09(a) of the Credit Agreement is hereby amended to read as follows:

(a) Consolidated Total Leverage Ratio. The Consolidated Total Leverage Ratio shall not at any time during any period set forth below exceed the ratio set forth opposite such period below:

Section 1.02 Amendment to Section 7.09(b) Effective as of the Effective Date, Section 7.09(b) of the Credit Agreement is hereby amended to read as follows:

(b) Consolidated Senior Leverage Ratio. The Consolidated Senior Leverage Ratio shall not at any time during any period set forth below exceed the ratio set forth opposite such period below:

ARTICLE II

Conditions Precedent

Section 2.01 Conditions Precedent. The parties hereto agree that this First Amendment and the amendment to the Credit Agreement contained herein shall not be effective until the satisfaction of each of the following conditions precedent:

(a) Execution and Delivery of this First Amendment. The Administrative Agent shall have received a copy of this First Amendment executed and delivered by each of the applicable Credit Parties and by all the Lenders and each of the conditions set forth in clauses (b) through (g) below shall have been satisfied.

(b) Representations and Warranties. Each of the representations and warranties made in this First Amendment shall be true and correct on and as of the Effective Date as if made on and as of such date, both before and after giving effect to this First Amendment.

(c) Notes, Loan Documents. The Administrative Agent shall have received all such Loan Documents as may be deemed necessary or advisable by the Administrative Agent.

(d) Resolutions. The Administrative Agent shall have received a certified resolution of the Board of Directors of each of the Borrower and Parent Guarantors authorizing the execution, delivery and performance of this First Amendment.

(e) Fees and Expenses. The Borrower shall pay the estimated fees, costs and out-of-pocket expenses incurred by counsel to the Administrative Agent in connection with the preparation, negotiation, execution and delivery of this First Amendment and all transaction contemplated hereby and thereby.

(f) Amendment Fee. The Borrower shall pay to the Administrative Agent for the account of each Lender executing this First Amendment a nonrefundable amendment fee in the amount of 0.10% of the sum of (i) such Lender’s Revolving Commitment plus (ii) such Lender’s Commitments under each Incremental Facility plus (iii) such Lender’s Facility Percentage of the Aggregate Outstanding Term B Loan Balance.

(g) Other Documents and Instruments. The Administrative Agent shall have received, in form and substance satisfactory to the Administrative Agent and its counsel, such other documents, certificates and instruments as the Administrative Agent shall require.

ARTICLE III

Representations and Warranties

Section 3.01 Representations and Warranties. To induce the Administrative Agent and the several Lenders parties hereto to enter into this First Amendment and to grant the amendments contained herein, each of the Borrower and the Parent Guarantors represents and warrants to the Administrative Agent and the Lenders as follows:

(a) Authorization; No Contravention. The execution, delivery and performance by the applicable Credit Parties of this First Amendment have been duly authorized by all necessary partnership, corporate or limited liability company action, as applicable, and do not and will not (i) contravene the terms of any Charter Documents of

any Credit Party, (ii) conflict with or result in any breach or contravention of, or the creation of any Lien under, any document evidencing any Contractual Obligation to which any Credit Party is a party or any order, injunction, writ or decree of any Governmental Authority to which any Credit Party is a party or its property is subject, or (iii) violate any Requirement of Law.

(b) Governmental Authorization. No approval, consent, exemption, authorization or other action by, or notice to, or filing with or approvals required under state blue sky securities laws or by any Governmental Authority is necessary or required in connection with the execution, delivery, performance or enforcement of this First Amendment.

(c) No Default. After giving effect to this First Amendment, no Default or Event of Default exists under any of the Loan Documents. No Credit Party is in default under or with respect to (i) its Charter Documents or (ii) any material Contractual Obligation of such Person. The execution, delivery and performance of this First Amendment shall not result in any default under any Contractual Obligation of any Credit Party in any respect.

(d) Binding Effect. This First Amendment and the Credit Agreement as amended hereby constitute the legal, valid and binding obligations of the Credit Parties that are parties thereto, enforceable against such Credit Parties in accordance with their respective terms, except as enforceability may be limited by applicable bankruptcy, insolvency, or similar laws affecting the enforcement of creditors’ rights generally or by equitable principles of general applicability.

(e) Representations and Warranties. The representations and warranties set forth in the Credit Agreement and the other Loan Documents are true and correct in all material respects on and as of the date hereof, after giving effect to the amendments contemplated in this First Amendment, as if such representations and warranties were being made on and as of the date hereof.

ARTICLE IV

Miscellaneous

Section 4.01 Ratification of Loan Documents. Except for the specific amendments expressly set forth in this First Amendment, the terms, provisions, conditions and covenants of the Credit Agreement and the other Loan Documents remain in full force and effect and are hereby ratified and confirmed, and the execution, delivery and performance of this First Amendment shall not in any manner operate as a waiver of, consent to or amendment of any other term, provision, condition or covenant of the Credit Agreement or any other Loan Document.

Section 4.02 Fees and Expenses. The Borrower and the Parent Guarantors jointly and severally agree to pay on demand all reasonable costs and expenses of the Administrative Agent

in connection with the preparation, reproduction, execution, and delivery of this First Amendment, and any other documents prepared in connection herewith or therewith, including, without limitation, the reasonable fees and out-of-pocket expenses of counsel for the Administrative Agent.

Section 4.03 Headings. Article, section and subsection headings in this First Amendment are included herein for convenience of reference only and shall not constitute a part of this First Amendment for any other purpose or be given any substantive effect.

Section 4.04 Applicable Law. THIS FIRST AMENDMENT SHALL BE GOVERNED BY, AND SHALL BE CONSTRUED AND INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO CONFLICTS OF LAW PRINCIPLES.

Section 4.05 Counterparts and Effective Date. This First Amendment may be executed in any number of counterparts and by different parties hereto in separate counterparts, each of which when so executed and delivered shall be deemed an original, but all such counterparts together shall constitute but one and the same instrument; signature pages may be detached from multiple separate counterparts and attached to a single counterpart so that all signature pages are physically attached to the same document. This First Amendment shall become effective when the Administrative Agent has received counterparts of this First Amendment executed by the Borrower, the Parent Guarantors, each of the other Guarantors and the Lenders and each of the conditions precedent set forth in Article II above has been satisfied (the “Effective Date”).

Section 4.06 Affirmation of Guarantees and Mission Loan Documents. Notwithstanding that such consent is not required thereunder, each of the Parent Guarantors and the other Guarantors hereby consent to the execution and delivery of this First Amendment and reaffirm their respective obligations under each of their respective Guaranty Agreements. Furthermore, notwithstanding that such consent is not required hereunder, each of Borrower and Guarantors (as defined in the Mission Credit Agreement) hereby consents to the execution and delivery of this First Amendment and reaffirm their respective obligations under each of their respective Mission Loan Documents.

Section 4.07 Confirmation of Loan Documents and Liens. As a material inducement to the Lenders to agree to amend the Credit Agreement as set forth herein, the Borrower, the Guarantors and David S. Smith hereby (i) acknowledge and confirm the continuing existence, validity and effectiveness of the Loan Documents to which they are parties, including, without limitation the Security Documents and the Liens granted under the Security Documents, (ii) agree that the execution, delivery and performance of this First Amendment shall not in any way release, diminish, impair, reduce or otherwise adversely affect such Loan Documents and Liens and (iii) acknowledge and agree that the Liens granted under the Security Documents secure (A) the payment of the Obligations under the Loan Documents in the same priority as on the date such Liens were created and perfected, and (B) the performance and observance by the Borrower and the other Credit Parties of the covenants, agreements and conditions to be performed and observed by each under the Credit Agreement, as amended hereby, and the Mission Credit Agreement.

Section 4.08 References to the Credit Agreement. Upon and during the effectiveness of this First Amendment, each reference in the Credit Agreement to “this Agreement”, “hereunder”, or words of like import shall mean and be a reference to the Credit Agreement, as amended by this First Amendment.

Section 4.09 Final Agreement. THIS FIRST AMENDMENT, TOGETHER WITH THE CREDIT AGREEMENT AND OTHER LOAN DOCUMENTS, REPRESENTS THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

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IN WITNESS WHEREOF, the parties hereto have caused this First Amendment to be duly executed and delivered by their proper and duly authorized officers effective as of the Effective Date.