ETROIT, Jan. 27  Hybrids, vehicles that save gasoline by combining electric motors with internal combustion engines, are emerging as the first alternative-powered cars to show signs of catching on with automakers and some consumers since the automobile's early days.

Toyota and Honda are already selling tens of thousands of hybrids, and General Motors and Ford, worried about ceding another fast-moving market to the Japanese, have announced plans to join them. The hybrid's rise has been encouraged by pressure from environmentalists and regulators, particularly California rules curbing greenhouse gases and smog-forming pollutants.

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"Hybrid technology is one that has great appeal because we don't have to really invent anything; we know they work," said William Clay Ford Jr., Ford's chairman, in a recent speech. "If these vehicles don't get customer acceptance, I really don't know what we do next."

A hybrid's battery is recharged by the internal combustion engine and by collecting energy when the car brakes. The battery powers an electric motor that supplements, or takes over for, the gasoline-powered engine. In the Honda Civic hybrid, an electric motor assists when the car is climbing hills or accelerating sharply. In the Toyota Prius, the electric motor takes over at low speeds. In both, the gas engine shuts off when the car stops.

Hybrids have until now been something of a curiosity and account for a small fraction of overall sales. Only three models  all small cars  are available, one from Toyota and two from Honda, and they cost a few thousand dollars more than conventional cars. About 150,000 have been sold worldwide since hybrids were introduced in the late 1990's, fewer than the number of vehicles typically produced by a single auto factory in a year.

But carmakers now appear ready for a much broader rollout. Earlier this month, at the North American International Auto Show here, G.M.  previously the industry's most vocal skeptic  publicly embraced the technology. The company said it would sell a hybrid version of its Saturn Vue sport utility vehicle in 2005 that would approach 40 miles a gallon in fuel economy, compared with mileage in the low 20's for current models. G.M. said it would offer vehicles with more limited forms of hybrid power, too, promising 10 to 15 percent improvements in fuel economy on four other models by 2007.

Also at the auto show, the annual beauty pageant where the industry trots out its latest designs and biggest pronouncements, Toyota said it would sell the first luxury hybrid, a Lexus sport utility vehicle, starting next year  part of a plan to sell 300,000 hybrids annually by mid-decade.

Ford plans to sell what will probably be the first hybrid sport utility vehicle, a version of the Escape, at the end of this year, and showed off a new hybrid prototype called the Model U.

Even the Army, which pays as much as $400 a gallon in battlefield fuel costs, had a hybrid on display  a hulking diesel combat vehicle, built by G.M., that is one of several prototypes being considered for service within a few years, including hybrid Humvees.

"You run those things on battery power; there's no noise," said Maj. Gen. Ross Thompson III, the head of the army's Tank, Automotive and Armaments Command, explaining the appeal of hybrids for the military. "For a reconnaissance mission, or if you want to not be noticed, you can use the batteries."

A century ago, in 1903, gasoline-powered Oldsmobiles shouldered past steam-powered Locomobiles to become America's top-selling brand. Never again would electric or steam cars rule the road. There is scant suggestion that hybrids may replace gasoline-powered cars in the same way. Among other things, two motors cost more than one.

But Stephen Girsky, an auto analyst at Morgan Stanley, predicts that hybrids could grow to 10 to 15 percent of American vehicle sales, which approached 17 million last year. Government incentives, gas prices and how much manufacturing costs can be reduced will be important factors, he said.

John Casesa, an analyst at Merrill Lynch, said that because the Japanese "view this as a core technology over the next decade," domestic automakers have to respond. "Inevitably, we're moving toward a future with higher fuel economy standards, risk to energy supplies and higher environmental consciousness," he said. "So there's a market pull here."