The backdrop is New York’s Hudson Valley, but I’m flying autobahn-style in the Mercedes S65 AMG, the world’s fastest four-door. A $195,000 version of Mercedes’ flagship sedan (Read “Behind the Line: Mercedes-Benz S-Class”), the S65 features a hand-built, twin-turbocharged V-12 engine capable of putting out 604 horsepower. And it has something equally indulgent and high-tech as well: its seats.

Mercedes’ Drive Dynamic seats are perhaps the industry’s most advanced: adjustable in myriad ways—including massage settings—and capable even of launching preemptive safety measures before a collision. But the Drive Dynamic seats are hardly anomalous; they are a leather-bound testament to the growing emphasis on interior and seat design in automobiles as carmakers look for every edge in a brutally competitive market. Luxury brands in particular have been adding everything from gimmicky gizmos to plush finishes to advanced safety features.

A car owner “spends the majority of time inside the vehicle,” says Lars Galley, of DaimlerChrysler’s customer research department. “Compared with the exterior styling, the interior has more time to make an impression on the driver.”

The new BMW 3-Series convertible delivers a technological first: seats, armrests, and a shift knob infused with reflective polymers that can reduce the leather’s surface temperature by as much as 36 degrees. Say goodbye to flash-fried thighs on sunny top-down days. And today’s techno-laden Lexus LS 460L and its hybrid sister sedan, the LS 600hL, offer a rear seat with airliner-grade reclining, along with a motorized footrest and shiatsu massage.

And there’s that Benz. Its Teutonic thrones have 15 separate pneumatic chambers that allow me to precisely tailor the amount of support from my thighs to my shoulders. Four levels of massage—slow-and-vigorous is my favorite—are designed to soothe muscles and fight fatigue. Linked to the stability control system, which monitors g-forces to prevent skids and rollovers, the seats can be programmed to grip your body more tightly through fast corners. (That function proves more distracting than helpful, especially at triple-digit speeds). Naturally, a flick of a switch turns the seats toasty-warm or frosty-cool.

Mercedes’ seats aren’t about just soothing your behind, they may possibly save your neck. They’re linked to a collision-avoidance system that uses radar to monitor traffic and springs into action when it senses the driver can’t stop in time to avoid a crash. In the moments before impact, seat belts tighten. Front passenger and rear seats shift to optimal crash positions. Seat bolsters inflate to hold occupants in place and boost the performance of seat-mounted side airbags.

It’s practically sci-fi stuff, especially when you consider that most cars sold in the U.S. didn’t even offer standard front seat belts until the mid-1960s. Volvo created the first modern, three-point seat belt on its PV 544 sedan for 1959. Then, with the science of ergonomics in its infancy, Volvo began working with medical experts to develop more-spine-friendly seats. Modern drivers might offer a two-aspirin toast to the 1964 Volvo Amazon, which pioneered the world’s first adjustable lumbar support (though you needed a screwdriver to operate it).

Volvo, now a division of Ford, is still obsessed with seats, so much so that it has two separate design groups: 11 engineers who work exclusively on front seats, and nine more for the back. Its seats consistently rank among the industry’s best in safety testing, while offering near-orthopedic support and Scandinavian style to boot. Company engineers note the challenge of creating seating positions that pamper and protect occupants who range from less than five feet to well over six feet tall. One backseat solution hits the market in October: Volvo’s new XC70 Cross Country features built-in child booster seats that rise to two different positions to accommodate children of various sizes.

Today’s smarter interiors and seats may be especially critical for a struggling Detroit. Many domestic designers freely acknowledge that they’ve been working to repair the damage caused by their reputation for poorly crafted interiors, including seats that often lacked the aesthetics and support typical of European brands.

Over the past two years, General Motors has shown the most Motown mojo. Cadillac seats, for example, offer top-grade, semi-aniline leather, with hand-applied stitched-leather and synthetics on doors and dashboards—supplied by the U.S. division of Germany’s Draexlmaier, which has crafted interiors for the likes of Bugatti and Mercedes’s Maybach.

Ed Welburn, General Motors’s chief designer, emphasizes the importance of seats to the car’s interior design. “It all begins with the structure, the support, the right profile to the seat,” says Welburn, only the sixth design chief in G.M.’s nearly century-long history. “We love heated seats and all those toys, but if the basics aren’t there, it’s not going to be comfortable over long distances.”

A trio of G.M. crossover S.U.V.’s—the G.M.C. Acadia, Saturn Outlook, and Buick Enclave—combine stylish cabins with a rarity: a third-row seat that actually accommodates six-foot-plus adults. “Everyone wants more and more interior space, and to give it to them without giving up comfort is a challenge,” Welburn says.

Making seats greener—we’re talking the environment, not upholstery color—is another fertile area for research. Seats in the 2008 Ford Escape and Escape Hybrid S.U.V.’s are wrapped in 100 percent recycled polyester fabric obtained from soda bottle resin; the 2008 Ford Mustang will use a soy-based seat foam that saves energy.

There are also design changes that are being mandated by the federal government. Starting in 2007, every car sold in America has included front passenger seats that can detect whether a small child or a heftier adult is occupying the spot. If the passenger is lightweight, dual-stage airbags launch more gently, lessening the chances of airbag injury. For 2009, new regulations will boost headrest standards for all cars.

Mercedes, for its part, sees seat design as a key element in Driver Fitness, its two-decade program to reduce stress and fatigue behind the wheel while promoting alertness, control, and safety. After monitoring drivers in a wide range of driving conditions, the company determined that S-Class test drivers’ average heartbeat—a key stress indicator—dropped by nearly 1 beat per minute compared with the average for drivers of the previous-generation model and by more than 7 beats per minute compared with drivers of S-Class vehicles built from 1979 to 1992.

That may be a historical about-face. Luxury cars were always supposed to make hearts race; now Mercedes wants to slow them down.