The Fairfax County Board of Supervisors, trying to extricate itself from the political and legal thicket it landed in last summer, last night turned its back on two subdivisions that earlier had been promised relief in a rezoning matter.

After almost two hours of discussion, the board voted 6 to 3 not to exempt the western Fairfax communities of Pleasant Hills and Pleasant Valley from a major downzoning it enacted last summer. The downzoning, affecting about 100 square miles of the county, is intended to restrict growth and so protect the quality of water in the Occoquan Reservoir.

Several hundred residents of the two communities, along with a developer who owns about 300 unfinished lots in Pleasant Valley, had appealed to the board in July to exempt them from the downzoning, saying the restrictions would make it difficult for them to rebuild or renovate their homes and would reduce the value of their property. The elected supervisors acceded on the spur of the moment to their well-organized demonstration, leading one resident to cheer what he called a victory for democracy.

Within minutes of the residents' departure from the board room, however, several supervisors and county attorneys were warning that any exception would endanger the entire downzoning if the action is, as expected, challenged in court. The supervisors then staged one of their most remarkable late-night displays of procedural confusion, arguing about what they actually had voted to do, reconsidering the vote and then debating how they could reconsider their vote to reconsider.

Eventually the board voted to give the issue another airing last night, but residents of the subdivisions were not impressed. "If you have already made up your minds, as I fear you have, then you are making a mockery of the hearings process and the rights of the citizens to be heard," said subdivision leader Larry Palmer.

Supervisor Audrey Moore, one of the six to vote against the neighborhoods, said the board's plan to control growth would have been threatened by a different outcome. "You exempt some and not others," she said, "and what's different about these people from other landholders included" in the downzoning? "Once you start, it's like knocking over a row of bowling pins . . . Where does it stop?"

Supervisors Marie Travesky and Martha V. Pennino, who represent western Fairfax, and Board Chairman John F. Herrity voted in favor of exempting the communities.

In an earlier, closed session yesterday, the board selected Doreen S. Williams to lobby for the county before the Virginia General Assembly in Richmond this winter. Williams, 56, is a George Mason University Law School student who has represented several Northern Virginia school boards in Richmond for the past four years. She is married to former state delegate Carrington Williams, a respected attorney and former United States Senate candidate who has been active in the effort to promote Dulles Airport.