IT'S EXHAUSTING, costly and maddeningly hard to learn. Even among friends, it is played with a desperate ferocity, leaving the loser fuming and the winner hot as a cheap muffler in a traffic jam. It destroys rackets, cracks backs, mutilates facial bones and attracts fewer than one- half of 1 percent of American adults.

It's squash. And for a certain kind of turbocharged overachiever, it represents a yuppie daydream of fitness, prestige and social access -- an ideal formula for courting success.

At first glance, it seems an unlikely, even ungainly, status totem: Two guys armed with long scrawny rackets, locked in a plaster closet twitching like electroshocked ferrets in pursuit of a tiny rubber ball. But in a fitness-mad age in which every pudge-padded Oreo junkie has a set of shopping- mall barbells, squash is limited to the genuine elite. And in a culture where the worst vulgarian boob can be trained to dress for success, squash and its rituals remain so redolent of Haut Prep, so freighted with Brahmin noblesse, that the mere word sets off subtle tremors of caste-consciousness.

No wonder. America's 400,000 frequent players, over 90 percent of them male and mostly on the East Coast, embody such a blue-chip demographic profile that, by contrast, the average country club seems like a cheese line. They have the whitest collars in the national washload: One in five is a lawyer, one in six a banker or financial executive; 55 percent have graduate degrees and 58 percent have household incomes over $40,000.

No wonder either that here in Reagan City, what with the resurgence of WASP chic, squash is on the upscale upswing. "Washington is one of the fastest growing squash markets in the country," says promoter David Carr, president of the National Capital Squash Racquets Association. "Ten years ago there were five courts in the area. Now there are 65."

Much of that growth doubtless derives from the court-sport boom of the mid-'70s, spurred by the nationwide mania for racquetball. But Carr attributes it to creeping Ivy -- a decade's worth of immigration from New England gentry-mills where squash has traditionally flourished. "It used to be that Ivy League people didn't think of Washington as a place to come. It was just a sleepy, dull place." But nowadays, he says, a couple of years with the right federal agency can give a young professional a good shot at Wall Street. And they bring their racquets with them.

The game "attracts people who are competition-oriented, who are trying to get a start in their careers," Carr says. And, incidentally, who aren't allergic to the sociological luster: "Squash tends to have a cachet you don't find in, say, Nautilus or basketball."

That cachet, he says, is convertible to cash. Deals can be cut, valuable contacts established and reputations enhanc in the pricey camaraderie of the locker room.

"I don't know how many people actually do business deals" on the premises, says Larry Nelson, manager of the Washington Squash and Nautilus Club (WSNC) in Lafayette Centre, "but you can pick up a lot in the locker rooms. They do talk business a lot, talk about what's going on, what's in the paper. And they all listen to each other."

Still, few if any players take up the game out of mercenary motives. Petch Gibbons, 31, a commercial real-estate agent with Barnes, Morris & Pardoe Inc., finds it "the ideal workout -- you're always moving. I'm in a highly competitive, high-risk, high-reward business," and on the court, "I can let loose any aggravations I've built up." Not that it's exactly mellow: "The few guys I play with, we kid each other a lot -- we keep saying that we're just going to hit the ball around a little. When really it's a cutthroat game every time."

"THE GUYS" BEGAN TO LOSE their exclusive franchise on the game about 10 years ago during the nationwide bigenderal sports revolution when clubs began opening their courts to both sexes. Psychologist Lynn Dorman, 44, a single parent and practicing therapist, says, "From the outside, it looked like women weren't allowed to play." But an attorney friend introduced her to the Capitol Hill Squash and Nautilus (CHSN) club.

"We don't really cultivate the traditional image," says CHSN's general partner Paul London, and "we've got a very aggressive program to recruit women." Since many of them "perceive it as a men's sport," he created, at the club, the Center for Women's Squash ("It makes it sort of legitimate") with frequent intramural tournaments. It raised the capital consciousness sufficiently so that 300 women now play there, including Dorman. "The women who play squash are different," she says. "A couple of people I played with later on lost interest. They wanted to play squash for fun -- they said they had enough competition during the day."

Brigid Quinn, 38, an executive assistant to D.C. City Council member John Wilson, first took a shot at the game a year and a half ago. She was dismayed first by the difficulty, then by the attitude of other women. The friend who introduced her to the game dropped it, and another friend "bought the shoes, the clothes and took a bunch of lessons," but then quit. Why? "I think she thought it was getting too competitive."

Which suggests one of the more subtle career- boosting byproducts of the game. Tell the boss you play squash, and he may rightly infer other desirable qualities: appetite for rivalry, ability to persevere while in pain and unrelenting concern for fitness.

IF THE GAME'S IMAGE is changing, its physical demands are not. A good amateur club player running at full crank can burn around 600 calories an hour in a frenzy of lunges, jerks, swats and twitches that make Michael Jackson look like Sitting Bull. (Many squash types disdain the langorous, amiable pace of tennis as tantamount to an admission of senility.) Because it uses both upper and lower body muscles and sucks up a huge amount of oxygen, squash is one of the fastest chow-burners, ranking with competition running, rowing, bicycling and cross-country skiing.

All of which blatant animality might seem to be at odds with the game's blue-blood reputation. Still, squash meets almost every criterion for cultural top-doggery.

Social critic Alison Lurie defines a high-status sport as "one that requires a great deal of expensive equipment or an expensive setting or both; ideally, it will use up goods and services rapidly." So does a court wall, which will eventually turn the toughest racquet to a litter of splinters. (Pros rarely approach a match with fewer than three racquets.) Replacements can be had for as little as $15 -- for the merest wretched stump from fabled Taiwan. But no serious player will be seen with less than a $50 Slazenger; and most favor the new uber-tech masterpieces of graphite fiber and boron at $100 and up.

Admittedly, the game does not encourage the sort of sartorial ostentation associated with, say, racquetball, which has generated rack-loads of lurid sportswear. Customary attire at Washington clubs is confined to demure T-shirts, frayed tennis jerseys and, of course, classic whites for the vrai prep look. (This restraint is in keeping with the game's self- effacing Yankee ambiance and monstrously courteous Briticisms. For example, one signals interference by saying, "Let, please" -- too genteel an utterance for someone duded up in a scarlet racquetball shirt wth black satin stripes.)

However, the squash ball's one-inch diameter accords nicely with the sports-status calculus devised by author Lisa Birnbach: The smaller the ball, the bigger the prestige. Nor does the game tend to produce injuries of the unseemly proletarian sort. Aside from the ankle, thigh and foot problems endemic to fast-break sports, a bit of tennis elbow or trochanteric bursitis from smacking the wall with a hip, the worst hazards are facial lacerations from the opponent's racquet or eye damage from the ball: Traveling routinely in excess of 100 mph, it can rupture the eye tissue and cause hemorrhage, paralyze the iris or pop the retina right off the back of the eyeball. Nonetheless, very few players wear safety goggles.

And when it strikes solid flesh (by a painful perversity of the rules, a player who is hit with the ball loses the point), it leaves a grotesquely distinctive bruise the size of a half-dollar and the color of used motor oil. Sports-injury specialist Dr. Hans Kraus has also noted a high incidence of acute back pain -- almost invariably from tense and harried businessmen who omit to warm up before pouncing onto the parquet.

Squash's most conspicuous consumption, however, arises in simply learning the game -- at about $8 per 45-minute period. "The problem," says Quinn, "is that you have to devote a lot of time to it." For Kevin Haggerty, 33, a former college jock and now an accountant in first-class shape, that meant "about a year to learn -- to get confident that you'll hit the ball every time you swing at it." The gravity of that commitment keeps the squash ranks small, compared with other racquet sports. "Special people find a challenge in squash that they don't find in racquetball," says Darwin Kingsley III, executive director of the U.S. Squash Racquets Association in Bala Cynwyd, Pa. "Racquetball is simply not as intense a game."

The difference between the two is analogous to that between checkers and chess. In racquetball, played on wide handball courts, shots are legal off the floor, any wall and the ceiling; racquets are mercifully stubby and wide (easing hand-eye coordination), the tennis- sized ball is easily visible and hyper-bouncy. As a result, virtually anyone with a set of Keds and sufficient motor skills to scratch his nose can play a plausible game immediately.

In the smaller squash court, the ceiling is out of bounds, as are foul areas on the front and back walls. The ball is about an inch in diameter and purposely lethargic. (Dropped from the same height, an American squash ball will bounce less than half as high as a tennis ball. Deader yet is the English version, the dreaded "black marshmallow" often used here in hot-weather play.) The racquets are tennis-length, but elegantly slender, with a strung face about seven inches wide. And whereas racquetball rewards brute power, squash is a game of meticulously acquired lobs and spins, precise blasts and gentle front-wall nicks.

"People do not switch from squash to racquetball,' says London of Capitol Hill Squash and Nautilus. "They trade up." And if they play longer, "the reason is, it doesn't get boring."

HENCE the vast difference in the audience for the two games. Squash players are older than racquetball players: 28 percent are aged 26-35; 25 percent from 36-45; and one in eight is 46 to 55. (Even the legendary Hashim Khan, multiple grand champion and sire of the Pakistani clan that dominated the game for decades, didn't hit his professional stride until he was in his thirties; ditto for such domestic greats as Victor Niederhoffer, the son of a Brooklyn policeman who devastated a bevy of prepsters on his way to the top.) Over half of all players have been playing for four to 15 years, and one out of 10 for over 20. Twenty-two percent of them learned to play in college.

For most, that has meant East Coast schools in or near the few major squash centers: Boston, New York, Philadelphia, Toronto Washington, which benefits from a large European population that learned the game at home. Outside those areas, the game is making only small inroads. "In the Midwest," where Nelson worked before coming here, "nobody plays -- I know of three courts in Kansas City."

THE CHIEF REASON for that -- and for the game's enduring prestige -- lies in its ironically aristocratic history.

Squash derives from the now-defunct British game of rackets, whose precise origins are obscured in the mists of antiquity. Squash pro Frank Satterthwaite, in his book on the game (The Three-Wall Nick and Other Angles, Holt, Rinehart and Winston) traces it to the late 18th century at London's Fleet Street debtor's prison where inmates, already bound by the necessary walls, made a pastime virtue of architectural necessity. They devised racquets and took to whacking the ramparts with a tightly bound ball of leather and cloth strips. (Thus, perhaps, scoring the first gaol in sports history.)

Soon the game was being played on tavern walls and then somehow, through an inexplicable transformation, became the rage of young nobles. At Harrow, Britain's venerable "public" school, it was so sought-after that those bored with waiting for court time developed a warm-up court and a special ball: a "squashy" India-rubber version with a hole in it.

throughout the second half of the 19th century at gentlemen's clubs, but gradually "soft or squash ball rackets," derived from the Harrow warm-up game and played on a much smaller court, became even more popular than its predecessor. The term "squash" first appeared in 1889, referring to the ball itself; but 10 years later the Westminster Gazette was using the word alone to denote the game, which by then had crossed the Atlantic and was a favorite of collegians.

Americans took to the game in earnest, holding the first U.S. tournament in 1907 -- 15 years before its British counterpart -- and evolving the heavier ball and smaller court that still distinguish the American game from the English version common in the rest of the world.

DESPITE its growing audience, the game remained the exclusive property of private clubs until 1973, when the first pay-for-play courts were built in the United States. Ten years later, Washington has courts at the Pentagon, the Arlington and downtown YMCAs, at clubs on Capitol Hill and Lafayette Centre, at several Courts Royal fitness emporia and at sundry exclusive venues such as the University Club. Not to mention two at the Federal Reserve and one at the Arnold & Porter writ-works, with more to come at the Army-Navy and Bethesda Health & Racquet clubs.

Consequently, the game now finds its traditional patrician image imperiled by its own appeal -- and by the mercantile metamorphosis that is changing the venerable institution of the gym into a screwball melange of exer- tech gizmos, juice bars and dating services. Although about 200 new sports clubs open in the United States every year, almost all of them containing at least racquetball courts, the days of the traditional racquet club are numbered, according to John McCarthy, executive director of the International Racquet Sports Association in Brook line, Mass.: "It's no longer a racquet sports industry. The clubs are all adding other products: Nautilus, aerobics, swimming pools, indoor tracks. It's a much broader phenomenon now," and "social programming is now as important as the athletic and fitness programs." (Even the downtown YMCA recently began advertising group excursions to the Bahamas.)

Which poses a dilemma for club managers: They need a broad clientele for revenue, but not so broad that it will drive away the more clannish squash purists.

The Washington Squash and Nautilus Club is an apt case in point. It opened as a squash-only operation in November of 1979 to initial "you just can't make it any more on a single sport," says manager Nelson, who was brought in a year ago when the local owners decided to get Spalding Sports Management to run the place. Among the problems: Squash traditionally has an eight-month "eason" (a vestige of pre-air- conditioning days), and when play drops off radically in the summer, so does income. So WSNC added Nautilus and aerobics, and "our average age dropped 10 years in the last six months," says Nelson. Still, he knows he has to "try to maintain a high-caliber squash club" for his cash cows, who in turn will be enhancing their financial status by using the club.

Squash spokesman Carr believes that the right mix of services can resolve the dilemma. "The modern elite squash player" he says, "has a well-developed social conscience and wants it to be a public facility. But yet he also wants to be part of this elite thing," and not forced into a lumpen hoard of mere aerobic bouncers and Nautilus crazies.

Carr, raised in an unassuming hamlet in northern New Jersey, says, "we were all brought up to think that there was something special about those people who went to the club for lunch. And subconsciously, we all want to be part of that club."