DETROIT (AP) _ General Motors Corp. opened a new round of auto pricing competition Thursday by announcing 7.9 percent financing on some of its cars and trucks, and Ford Motor Co. quickly followed suit with the same incentives.

GM, the No. 1 automaker and industry price leader, announced it would offer the rates on roughly 40 percent of its cars. Ford announced a few hours later that it, too, would offer 7.9 percent financing on about 40 percent of its cars.

''Essentially, it is a competitive response to help us maintain our sales momentum,'' said John Seig, spokesman for No. 2 Ford.

GM sparked a cut-rate loan and rebate war during August and September, causing domestic auto sales to double overnight. The automaker, however, lost market share during the pricing war with other domestic automakers.

In an interview with the The Associated Press on Dec. 17, GM Chairman Roger B. Smith said GM would be more careful about launching its next sales incentive campaign, and that the company would even prefer that one of its competitors take the first step.

But GM spokesman Harold Jackson said competitive considerations prompted the biggest automaker to move first.

''It's a very competitive industry,'' he said. ''I don't think that what (Smith) said was meant to last right into the future. We do have good supplies of these vehicles and are anxious to get off to a fast start, and it was felt that this program should be launched.''

GM has been losing market share to Ford and Chrysler Corp. for more than a year. Its dealers have an 110-day backlog of unsold cars compared with the desired level of 60 days, said Arvid Jouppi, an auto industry analyst based in Detroit.

''The consumer has again indicated that he or she will not pay the current prices,'' Jouppi said. ''The consumer is dictating to the manufacturer the future of the market.

''I think it means we're going to have this kind of marketing irregularities for the rest of the model year.''

In the most recent sales reporting period, the first 10 days of December, GM reported a 10.2 percent decline in sales from the same period in 1984, while Ford sales were down 18.1 percent and Chrysler sales were off 11.4 percent.

Chrysler, the No. 3 domestic automaker, has no plans to replace its current incentive program, which features 8.6 percent financing and rebates of $500 to $1,000 on some cars, said Chrysler spokesman B.F. Mullins.

''Nothing is planned; GM's just catching up,'' he said.

Both Ford and GM said the 7.9 percent rates would be offered on deliveries taken from existing inventories through Feb. 22.

''We expect this highly attractive 7.9 percent financing, together with the good stocks dealers have of most new 1986 GM cars and light trucks, to give GM a fast sales start in the New Year,'' said James G. Vorhes, vice president in charge of GM customer sales and service staff, said in a statement.

GM said that among the cars covered by the 7.9 percent financing is the slow-selling Chevrolet Nova, built jointly in California by GM and Toyota Motor Corp. of Japan.

Most of the models included in the incentive program have four-cylinder engines, the automaker said.

Other vehicles include some models of the Chevrolet Cavalier and Celebrity; the Pontiac Fiero, Sunbird, 6000, Grand Prix and Bonneville; the Oldsmobile Cutlass Ciera and the Cutlass Supreme; Buick Somerset, Skylark and Century; and the Cadillac DeVille.

Trucks in the program include the Chevrolet El Camino and GMC Caballero.

Ford models eligible for the low-interest financing include 1985 and 1986 Tempo and Topaz, 1985 and 1986 Thunderbird and Cougar models with 3.8 liter engines; 1985 Merkur XR4Ti; 1985 F-150 4X2 regular cab and supercab models; and 1985 and 1986 Ranger 4X2 regular cab and supercab models, except for the Ranger ''S'' model.