San Francisco, California, and the Motor City used to be the perfect couple.

With its freeways and easygoing, put-the-top-down mentality, the Golden State was the car state of the nation – and the car was most definitely made in America. Can you imagine the Beach Boys singing about imports? ("And she'll have fun fun fun 'til her daddy takes the Honda Civic away.")

Or try to envision Hollywood icons of days gone by – McQueen and the Rat Pack – in anything but the likes of Fords, Lincolns or Chevys? Of course not. But the freeways got clogged, American cars lost their ring-a-ding-ding and the romance soured. California remains the car state, but about the only new domestic sedans you see on the roads now are from the rental car lots of Avis, Hertz and Budget.

For three decades, Detroit and the Golden State have been accumulating irreconcilable differences. And lately, like all failing marriages, the lawyers have become involved.

Trouble comes on two fronts. First, the state's regulators and environmental groups have been fighting the automobile industry on air quality for years and have become a far more aggressive regulatory forum than Washington. The industry is expected to sue California over its plan to become the first state to regulate tailpipe emissions of gases that are linked to climate change.

Second, and even worse for Detroit, is apathy from California consumers, who buy more than one out of every nine vehicles sold in the United States. The traditional Big Three domestic automakers – General Motors, Ford Motor and the Chrysler Group, a division of DaimlerChrysler of Germany – still sell plenty of sport utility vehicles and pickups here. But the sales challenges that the Big Three face nationwide are far worse in California, the nation's largest auto market and an important trendsetter.

"In California, people used to write songs about T-Birds and Corvettes," said William Clay Ford Jr., the chairman and chief executive of Ford, at an industry conference last year. "Today they write regulations."

Actually, the Big Three no longer exist as such in California, which trails only Connecticut and Washington D.C. in the percentage of import sales. Nationally, G.M., Ford and Chrysler still rank as the top three automakers by volume, even though their market share fell to a record low in August.

But not here.

In California, Toyota is the top-selling brand, though nationwide it is third, behind Ford and Chevrolet, according to R. L. Polk & Company, a consumer marketing group in Southfield, Mich. Honda is the state's third- most popular retail brand, behind Ford.

Among automakers, the Chrysler Group, which includes the Chrysler, Jeep and Dodge brands, ranks fifth in California, but it is third nationwide. In the state, G.M. has 18 percent of the retail market, a far cry from its 27 percent national share in 2002.

And if the Big Three command roughly 60 percent of the nation's auto market, more Asian vehicles than domestics are sold in California, according to Polk.

"The domestics think of California as another country, almost," said Lonnie Miller, the director for analytical services at Polk.

Indeed, said Gary Dilts, Chrysler's senior vice president for sales, "It's a long way from Detroit and it's certainly a different culture," adding that the company recently put a new marketing team in place in the state to try to revise its strategy.

"Vehicle trends develop and start in California, and those of us who live here like to think they move east," said Mike Jackson, G.M.'s regional general manager who oversees 15 states in the West.

G.M., which is still the world's largest automaker by volume, has embarked on an aggressive strategy of its own.

The company has held several auto shows throughout the state that let residents test G.M.'s vehicles against the competition, partly to get a second chance with customers who do not consider buying domestics.

"I like to say the road to greater share for G.M. nationally goes through California," Mr. Jackson said. "If we can find a way to get greater share here, we could get greater share nationally."

There are some ingrained disadvantages for Detroit in California, like the residents' affinity for companies from the Pacific rim. Not only is Southern California the base of North American operations for the Japanese Big Three – Toyota, Honda and Nissan – but the state also has large Asian and Hispanic populations that make for a high degree of diversity.

"You've got a very cosmopolitan audience, people coming from all walks of life and many other countries," Mr. Miller said. "People in L.A. have been around the block more times than people in Dubuque."

The state has a car-loving culture that craves new products, and in the last few years, Ford and Chrysler have been bleeding red ink, cutting costs and slow to put newly designed vehicles on the market.

Why is there so little interest in modern American cars? "All the cars look the same," said Janet Massey, a 54-year-old part-time bookkeeper who lives outside Fresno. With her husband, Brian, she has a 1965 Mustang and a 1967 Pontiac Firebird that she has owned since 1969. She sits on the board of a local classic car show, and her husband belongs to a car club called Pontiacs of Central California.

"It's hard to put your finger on what was so special about those cars in the 60's," Ms. Massey said, adding that today's cars have "no personality."

Chris Cedergren, an analyst at Iceology, a market research company based in Los Angeles, said, "Our offices are on the west side of L.A., and the only domestics you see are trucks – Tahoes, Chevy Suburbans. Other than that, they don't exist."

Light trucks — S.U.V.'s, pickups and minivans – remain an area of strength for the Big Three, but they still face increasing competition from overseas. Three out of 10 Hummers are sold in California – and the state's new governor, Arnold Schwarzenegger, has been Hummer's biggest celebrity enthusiast. As for cars, the strength of domestics has been in the marginally profitable fleet business. Ford, Chevy and Dodge command more than half the state's fleet sales, a presence that can reinforce an image of blandness.

Midwestern auto executives are also vexed by their perpetual feuds with California's smog-battling regulators and environmental groups. Last year, the state passed the nation's first legislation aimed at curbing automotive emissions of greenhouse gases like carbon dioxide, which are linked by many, but not all, scientists to global warming trends.

And this month, California planned to sue the Environmental Protection Agency over the Bush administration's decision saying that the agency did not have the authority to regulate greenhouse gases.

The state's plan to regulate automotive emissions of greenhouse gases has not been well received by Detroit, and a legal challenge – only the latest from the industry – is most likely.

An editorial cartoon in The Detroit News captured some of the industry's sentiment. It showed a Californian driving a car with a muffler bolted over his mouth. The caption read, "California's Regulating Carbon Dioxide From Automobiles." Two conservatively dressed Midwestern types watch from the sidewalk, with looks that seem to say "what will those crazy tree-huggers think of next?" (For the record, the state intends to regulate carbon dioxide coming from tailpipes, not from the mouths of its drivers.)

Detroit cannot afford to take the state's actions lightly. Because its clean air regulations predate the federal Clean Air Act of 1970, California was allowed to maintain its own tougher regulations. Since the state is the nation's largest auto market, this fact alone would make it formidable, but other states also have the option to follow California's air regulations instead of Washington's.

Northeastern states like Massachusetts, New York and Vermont do just that, giving the state effective regulatory control over one-fifth of the auto market.

In Washington, Midwestern Democrats frequently align with Republicans on regulatory issues, effectively blocking significant increases in fuel economy or emission regulations. But California has no Midwestern Democrats in its legislature, as well as a history of smog and a minuscule domestic auto manufacturing base, so it has become a much more solicitous regulatory battleground for environmental groups, many of which are based in the state.

One protracted battle recently ended when G.M. and Chrysler dropped a lawsuit over the state's Zero Emissions Vehicle mandate, which requires automakers to make millions of vehicles with low or zero emissions over the next couple of decades.

For 13 years, the state and the industry fought over the measure. But because auto production cycles take years, the Z.E.V. mandate, as it is known, forced the industry to produce a multitude of green car prototypes.

Last month, the French tire company Michelin sponsored a green car competition in Sonoma, Calif., and many of the vehicles on display could have partly attributed their existence to the Z.E.V. mandate.

As part of the event, dozens of cars of unusual shapes and sizes paraded across the Golden Gate Bridge, with the technologies depending on their origin. The Germans brought relatively clean versions of diesel cars, which are popular in Europe, and the Japanese brought hybrids – their preferred solution to reducing emissions and improving fuel economy. G.M. brought its fuel cell prototypes, including Hy-wire, a car that has no dashboard and has its entire drive system under the floor.

The state has "squeezed a lot out of that in terms of forcing the industry to invest in innovation," said Carl Pope, the executive director of the Sierra Club in San Francisco, referring to the mandate. "The sad thing is that everybody invested in it because they had to, but the only ones getting anything out of it are Toyota and Honda."

Mr. Pope was talking about the two companies' hybrids, cars that supplement their gas-fed internal combustion with electric power. So far, only Toyota and Honda have put hybrids on the road. The 2004 version of the Toyota Prius hybrid, which gets about 50 miles a gallon and qualifies as what California regulators call an advanced technology partial zero emission vehicle, or A.T.-P.Z.E.V., was one of the top performers in the competition.

California, though, could hardly be simplified as a tree-hugging state. In fact, the recent growth of light truck sales there has been slightly higher than the already voracious pace of the nation.

Nationally, light trucks account for more than half of all new vehicle sales, up from about a fifth in 1980.

"You could say California is talking out of both sides of its mouth," Mr. Miller of Polk said. "You have such a large pool of vehicle buyers that you can find a place for just about anything there."

Mr. Cedergren of Iceology thinks that environmental concerns have "not an ounce" of correlation to the Big Three's troubles in the state, and certainly not in Southern California.

"In L.A., it's all about image and fashion," he said. And that has not been a resonating feature of the modern American car.

Jim Jordan, 33, a T-shirt printer in San Diego, loves cars. He has a 1950 blue Mercury, a 1957 Chevy truck and a 1964 T-Bird. He belongs to a local car club, called the Spares, whose bylaws require that "all cars must be pre-`Summer of Love' " because "hippies killed the automobile."

So why do he and his wife also have a 1991 Mazda – for everyday driving?

"They last longer," Mr. Jordan said, adding that he felt that since modern cars were drained of style, he looked for dependability instead.

"You can have a 10-year-old foreign car and you know you're going to get 200,000 miles out of it. If you've got a domestic car with over 100,000 miles on it, you know you have to do stuff to it," he said. "You don't want to have your chin under the hood with the car your wife drives to pick up the kids."