Fairfax County Board Chairman John F. Herrity announced yesterday that he will seek a second term as the county's top elected official, saying he is "highly doubtful" that he ever again will run for Congress.

Herrity, whose acerbic conservativism has made him one of the Washington area's best known politicans, said "there are too many challenges for Fairfax County in the years ahead... to choose any other course of action but to seek reelection..."

The 47-year-old Republican, who last year lost a bitter race against Democratic Rep. Herbert E. Harris, said his defeat was "a message" from some voters that they wanted him to remain as head of the nine-member Board of Supervisors.

Democrats have yet to field a candidate for the chairmanship, but party leaders said yesterday they were not surprised by Herrity's announcement. "We knew he would run for something," said Emilie Miller, head of the Fairfax Democratic Party. "It was just a matter of him making up his mind."

Fairfax County, Herrity said, must cut its share of Metro transit costs, provide relief from rising real estate taxes and control the county's rapid growth.

Herrity said he is hopeful that the county will increase one item in its budget, the $15,000-a-year salary allocated for his job.

Although the position is classified as part-time, Herrity said he has been strained to support his family of six on the county pay plus the $20,000 a year he earns as an insurance salesman. He said the county should pay its chairman $17,000 next year.

Herrity, first elected to the Fairfax board in 1971 as the representative of the Springfield district, yesterday listed five accomplishments from his terms in office.

They were the county's action tapping the Potomac River as a water source, extending Interstate Rte. 66 from the Capital Beltway to the Potomac, keeping the county government payroll down, attracting new industry to Fairfax, and a law requiring that the covers of Playboy and Penthouse magazines be kept from public view in county stores.