The Maryland Thoroughbred Racing Board yesterday approved the state's 1982 racing schedule, which includes a 48-day summer meeting at Laurel.

Its actions were perfectly predictable, following the general pattern of recent years. But the board's hearing also bared the extent of the hostility between Maryland's thoroughbred and harness-racing interests.

Laurel's May 24-to-July 19 season will involve several weeks of head-on conflict with neighboring Freestate Raceway. What troubled the harness industry even more, though, was Timonium's request to conduct "twilight" racing, with a 3 p.m. post time, every weekday during its 42-day meeting.

Such late hours would be an intrusion into the province of harness racing, protested Freestate's attorney, Martin Jacobs.

Timonium's plan would siphon away customers from the night racing at harness tracks, said Ben Schwartz, chairman of the Harness Racing Board. "It could only result in a negative impact on the harness industry," he said.

But members of the thoroughbred industry were hardly shedding tears for their counterparts. Last spring the thoroughbred interests had worked together for the passage for the so-called track consolidation bill that would have been a windfall for them. It seemed likely to pass, but was stopped by a filibuster presumably orchestrated by harness-racing interests.

"These people talk about harmony and peace," complained Pimlico General Manager Chick Lang, "but then they try to bury you."

Fendall Clagett, head of the Horsemen's Benevolent and Protective Association, told the commission, "Whenever it comes to a question of harness racing infringing on the thoroughbreds, that's okay. It's done in the interest of the state of Maryland. Only when it's the other way around is it infringement. But when they asked for and got racing dates beyond their traditional summer dates, it meant a loss to the thoroughbred industry of $100,000 a day."

The thoroughbred board decided to defer its decision on Timonium's request until it could conduct a joint meeting with the harness board. The objective is presumably to keep a measure of peace in the family, but commission members seem inclined to approve the twilight racing. "If we try to cooperate with them," asked Kenneth Proctor, "what assurance do we have that they'll cooperate with us in the legislature?"

The racing schedule approved yesterday gives Laurel its turn to conduct the summer dates that were run at Pimlico this year and Bowie in 1979. In order to race during hot weather, Laurel will have to spend more than $1 million for an air-conditioning system that will cool only about half of the plant.

The board tried to pressure Laurel, Bowie and Timonium to make overdue physical improvements, saying that their date assignments could be revoked if the jobs weren't done. Laurel was ordered not only to install the air-conditioning system but also to rebuild two barns. Bowie was told to build five new barns in 1982. Timonium was told to install an elevator and submit plans for an air-conditioning system.

The racing dates: Bowie, Jan. 1-March 12; Pimlico, March 13-May 22; Laurel, May 24-July 19; Timonium, July 20-Sept. 6; Bowie, Sept. 7-Oct. 18; Laurel, Oct. 19-Dec. 31.