Labor contract negotiations between union leaders and managers of Maryland's major race tracks are scheduled to resume Thursday, sources said yesterday, under the threat of a strike that could jeopardize Pimlico's summer meeting.

Chick Lang Sr., Pimlico's general manager, refused to comment yesterday on the negotiations and their effect on the track.

Officials of Local 27, the United Commercial Food Workers' Union, which represents the workers, have said they will seek strike sanctions if the contract dispute is not resolved by Monday, the day Pimlico will reopen. The 600 track employes affected include mutuel clerks, security guards, jockeys' room valets, starting gate crew and ticket sellers.

Two face-to-face negotiating sessions have proved fruitless. In the first, Tom Russow, union president, rejected all of management's new contract proposals. In the second, management declined to bargain after Michael Milanowsky, its chief negotiator, quit unexpectedly. Reasons for his resignation have not been unannounced.

Track employes have worked without a contract since July 1 because Bowie is operating its final meeting, which will conclude Saturday.

Representatives of Laurel and Pimlico are bargaining for management, but each track has its own negotiators. The tracks have a profit-sharing agreement but could bargain separately.

Yesterday, Frank DeFrancis, Laurel's president, declined to comment on progress of the negotiations. However, he did say, "I think this is a very sensitive area right now. When you're having negotiations, any third party -- be it the media or anyone else -- can go ahead and dilute the success of the negotiations."

DeFrancis said he expects serious bargaining progress to be made before Friday. "Pimlico is going to open Monday," he said. "Something is going to happen in a very definitive way before then."