The National Highway Traffic Safety Administration has inflated several trial balloons in hopes of eventually getting air bags into domestically purchased cars.

The NHTSA is proposing to buy 5,000 new 1985-model cars equipped with air bags for the General Services Administration in an effort to give manufacturers an incentive to produce the safety devices. In October 1981, the NHTSA halted its own efforts to require air-bag use.

The agency said it now is trying to accomplish through incentives what it would not do by fiat. But most of the major domestic automakers interviewed yesterday say they aren't planning to take up the NHTSA offer.

A second approach would be to buy air-bag assemblies from smaller manufacturers and install the units in government-owned cars. NHTSA Administrator Raymond A. Peck said the agency will seek bids for this work, but said retrofitting could prove more costly than assembly-line installation.

Officials at Ford Motor Co. expressed enthusiasm for the NHTSA initiatives, but disagreed over whether the company could gear up in time to meet a 1985-model purchase order.

"If you said, 'Go,' we couldn't start up tomorrow on a project like this," said Robert H. Harnar, Ford's manager of technical information. "Probably the earliest we would be able to go would be for the 1986-model year, which means starting in the fall of 1985."

"I don't think Ford should be quoted as saying it would take until the 1986 model year to do what GSA wants," said Roger Maugh, Ford's director of automotive safety. He said, however, that NHTSA's fall-1984 goal might be attainable.

Just over half of the federal government's 1982 cars came from Chrysler Corp., and Chrysler officials say they would like to do that much, or better, in the future. But the air bag fleet proposal "gives us some pause," said Chrysler Washington spokesman Richard Muller.

Chris Kennedy, director of Chrysler's office of federal affairs, put it this way: "We don't think 5,000 cars would be enough to justify the research and development costs and the tooling costs" that would be involved in such a program.

"Any time someone is talking about a fleet purchase of 5,000 cars, we're interested," said Robert C. Stempel, general manager of the Chevrolet Division, the largest in General Motors Corp., which is the nation's largest automaker.

"But we don't have all of the particulars on this thing. We haven't been told very much about what the government expects," Stempel said.

What the government expects to do is to use the fleet-purchase tack to inspire state and local governments and large commercial concerns to follow the same example, said Peck.

"We're serious about this. We're doing nothing more than trying to guarantee a market," Peck said. He said the cost of putting air bags into federal cars could run as high as $500 a vehicle. The NHTSA would have to absorb all of that cost because the air bags aren't designed to save property, which is a GSA concern. The bags are designed to save people, which is a NHTSA concern, Peck said.