Incumbent Republican John J. Buckley won a close victory yesterday over Democratic challenger Mayor Walter J. Sullivan in the contest for Middlesex County Sheriff.

Unofficial returns from the Associated Press gave Buckley 230,597 votes or 51.4 per cent, to Sullivan's 218,234.

Democrat Sullivan was favored in the race, which was not decided until final returns were received at 5:30 a.m. yesterday. Sullivan conceded before noon yesterday.

Buckley won despite a three-to-one Democratic advantage in county registration and statewide Democratic victories. As expected, Buckley did well in normally Republican small towns.

Running on a platform of prison reform and strict gun control, Buckley also outran Sullivan in liberal areas. Middlesex County includes all of Cambridge and Boston's northern and western suburbs.

Sullivan--traditionally the biggest votegetter in Cambridge City Council elections--barely carried his home city, 14,869 to 14,329.

Both candidates said that Sullivan's long involvement in local politics cut into his Cambridge margin.

Sullivan said yesterday that as mayor, he was blamed for tax increases, although he personally has no direct responsibility for tax rates.

Sullivan conceded that "the liberal vote did a job" on him. He said Buckley's liberal support came from Framingham, Newton, Lexington and Concord.

Buckley attributed his victory to "the issues we stood for, such as gun control and inmate furloughs."

The victorious candidate said that he was encouraged by the success of an advisory referendum on gun control which was on the ballot in Wellesley, Newton, Revere and Winthrop.

Sullivan won Somerville by a greater than 2-to-1 margin, and carried Malden, Waltham and Lowell. Buckley took two-thirds of the Newton vote, also winning Lexington and Framingham.

Within Cambridge, Buckley did well in the Brattle Street area and the neighborhoods that include Harvard and MIT. Sullivan won overwhelmingly in East Cambridge, Central Square and North Cambridge.

Sullivan said yesterday that he would take the defeat "in stride."

"I'm beginning to campaign today for re-election to the City Council," he said