John F. Herrity, chairman of the Fairfax County Board of Supervisors, had a mild heart attack Thursday night, his third in the past nine years, an aide said yesterday.

Herrity apparently had the attack while working at his county office and drove himself to a nearby hospital, said the aide, Bob Foreman.

The board chairman, who will turn 53 this month, will be unable to return to his county duties or his job as an insurance salesman for about six weeks, according to Foreman.

Foreman said doctors are uncertain as to what caused the attack, but they said Herrity will be moved to Fairfax Hospital this week for further testing to determine whether bypass surgery would be necessary.

Doctors described the heart attack as "mild," Foreman said, adding, "On a scale of 1 to 10 with 10 being the worst they rated it a 1.5."

Herrity, a Republican serving his third term on the board, apparently had the attack shortly after meeting with D.C. Mayor Marion Barry and other officials over the controversial Lorton Reformatory in southern Fairfax.

He was working in his county office at the Massey Building in Fairfax City when "he felt a sharp pain in his arm," Foreman said. "He then drove himself to Commonwealth Hospital."

Herrity, who began a new dietary and exercise regimen after having two heart attacks nearly nine years ago, jogs four miles a day but has continued to smoke cigarettes, Foreman said.

Only members of his immediate family have been allowed to visit Herrity at the hospital, Foreman said.

Foreman said he talked with Herrity by telephone this weekend and that the board chairman sounded "real chipper."