About one-third of Prince George's County's 576 marked police cars have visited the repair shop in the past 18 months because their driver seats have deteriorated.

The problem, according to E. G. Mattera, manager of Prince George's fleet, maintenance garage, has cropped up in approximately 200 of the 262 Chevrolet Novas that the department purchased in 1978. Twenty of the Novas recently were out of commission at the same time because of broken seats.

The problem apparently stems from the car's bucket-seat design, Chevrolet and local police officials agree. No other make of Prince George's marked police cars reportedly has seat problems.

Prince George's is not alone in having trouble with the Nova's seat. A Montgomery County police official said his department owned 15 Novas, almost all of which had developed seat problems. Local police officials said police departments in Portsmouth, Va., Jacksonville, Fla. and San Francisco also have reported faulty seats.

The seats "are deteriorating in every way that you can think of," said Mattera. He said his mechanics find that often the seat simply breaks away from the car's floor, or the seat's adjustment tracks crack.

In other cases, he said, the support springs give way, "leaving the officer just about sitting on the floor," or the backs of the seats bend almost to the snapping point.

Thomas Rockstroh, regional fleet account representative for Chevrolet defended the Nova as the "finest cost-per-mile car for a police department that there is."

While conceding that the car's bucket-seat design might make it more vulnerable to problems, he insisted that it was not defective.

"You have to expect some rough use in the police business," said Rockstroh.

"That car is the policeman's office. It should be expected that there will be a maintenance problem.

"But, Rockstroh insisted, "that doesn't mean that there is a defect."

Rockstroh said Chevrolet developed the bucket seat for police cars because police departments around the country had requested space at the driver's side for sophisticated radio equipment, which the bucket seat provides.

Chevrolet offered the seats as a special equipment option on Novas sold to police departments between 1975 and 1978.

Prince George's police department officials said recently they thought the frame of the seat might break if the car were struck from behind and that the driver's back could be broken as a result.

Mattera said Chevrolet has supplied parts to solve the problem but that the devices failed to work as expected.

"We put in a reinforcement plate on the back of the seat," he explained. "The frame of the seat would just break somewhere else."

Having given up on repairing the seats, the county now plans to replace them all with an entirely different design. So far, 40 new seats have been installed and the county recently accepted bids on 200 more seats.

The seats are expected to cost about $125 each to buy and install -- about $32,750 for the entire fleet of 262 Novas.

County attorney Robert B. Ostrom this week said the county has informed Chevrolet that it expects the auto company to pay for the Nova seat repairs and replacements.

If Chevrolet refuses, the county will sue to recover its expenses, Ostrom said.