Time was, cool weather brought tennis to a screeching halt. Like baseball, tennis was regarded as properly an outdoor game, and the U.S. Open, traditionally played in New York in the suffocating heat of early September, signaled season's end.

Not so today. The pro tours either go to Australia and Hong Kong or take to the indoor courts. Tennis news comes at us 12 months a year. Tennis, like baseball in Houston and Seattle, has gone indoors.

Indoor tennis has come a long way. Before the popular boom took hold 10 years ago, players could beat the weather only in some drafty old armory with skinny lines painted on the varnished wood floors. Balls skidded on a surface slicker than a basketball court, giving big servers an awesome advantage. Lobbing was a lost art unless you could navigate the rafters and an occasional wayward pigeon. Lighting fixtures intended for military marching drills turned the court into a mirror of hot spots. The game was more crapshoot than a game of skill. In short, it was torture.

Nowadays, a trip to the indoor tennis club can be the highlight of your week: Some club lounges are more comfortable than your living room. Climate-controlled playing areas have soft indirect lighting, netting court dividers and playing conditions that never change.

Today's winter player is a pampered one, indeed. Carpeted locker rooms come replete with sauna, steam rooms, whirlpools and professional masseurs. The clubhouse may include wide-screen television, futuristic furniture, a bar, a restaurant and picture-window viewing areas for those whose experience of the game is chiefly vicarious.

One club even offers a purely social membership to complement the "full" membership (tennis and health facilities) and the "athletic" program (health spa only). For a low fee, the social members can sip daiquiris with friends or business contacts and watch tennis from a gazebo restaurant without ever touching a racket.

Indoor tennis facilities come in many sizes and shapes. A classic of the early days of private clubs is Allie Ritzenberg's Indoor Tennis on River Road in Bethesda. You'd never know there was anything so elegant as tennis going on inside the aging gray corrugated-metal warehouse where Allie put down two Astroturf courts 11 years ago. Even today it takes a personal guide to find the place. The courts, one unnaturally cramped by enormous I-beams to the side, are laid out in the old loading-bays beneath a crosshatching of rafters that almost qualifies as art deco. Maybe one reason Allie's place is so obscure is that his chief customers when he opened up -- the tennis-playing Kennedys and others of fame or fortune -- would just as soon no one knows where to find them.

A favorite of busy politician and media mogul alike is the Arlington Y Tennis and Squash Club. Relatively close in on the Kirkwood Road continuation of Spout Run in Arlington, this facility is no more like an old YMCA than a millionaire's Maine "cottage" is like your place at the beach. Arlington offers cushioned-surface courts, carpeted locker rooms with small weight rooms, a practice room, a nursery and large upstairs viewing areas. Its prices are as high as those at any private club ( $12 per hour at midday, $20 during prime evening hours), but it's the only club in the area open 24 hours a day. The true hard-core tennis fanatic can, for $75 per season (November-March), play his heart out for any length of time between 1 a.m. and 6 a.m. Thirty-five insomniac souls booked this "Starlight" program last year.

More typical of economy programs in many indoor clubs are Early Bird (6 a.m. to 9 a.m.) and Night Owl (10 or 11 to midnight) bookings. For a fixed fee (say, $60 membership plus $200 for the court time) a player can have a court on the fringes of the harried professional's work day and play any time he or she wants as long as a court is available. The early bird, for instance, is a favorite of Senators Lloyd Bentsen and Charles Percy at the Arlington Club.

Some tennis clubs resemble super-posh health spas with tennis on the side, like the Skyline Racquet and Health Club on Leesburg Pike in Falls Church. It's the ultimate in shopping-center tennis facilities, offering four glassed-in racketball courts, a steaming jacuzzi-style whirlpool beside a 25-meter swimming pool, a dance studio and two mirrored exercise rooms filled with assorted torture devices. ("One side for upper body, the other side for lower body," explains the heavily muscled weight-instructor.) Its spacious locker rooms include steam, sauna, sunroom and full-time massage experts. The main lounge is sprinkled with armchairs that look like stacks of chocolate bars facing a large color TV. And, oh, yes, there is tennis -- seven cushioned courts with a rubber-like surface.

Memberships to such a complex club come in three forms at various prices: tennis only, health spa only, or both.

There is still such a thing as pure tennis: This is the style at Cabin John Indoor Tennis on Democracry Boulevard in Bethesda. Celebrated head-pro and former Wimbledon champion Pauline Betz Addie and Allie Ritzenberg run a no-nonsense operation of six hard courts under a permanent roof. This was one of the first indoor tennis facilities in the area.

There is a distinct shortage of indoor tennis courts within the District. Five of the public hard courts in East Potomac Park are covered every winter with a noisy airblown vinyl bubble for indoor play. The cracking and sloping problems during subway construction over the past two years have now been corrected and the courts resurfaced, says the management. The rates at East Potomac -- or Hains Point, as regulars call it -- are lower than in private clubs ($8.50 to $14) but the amenities are simple: postage-stamp sized locker rooms, a trailer-like waiting area, beer and sandwiches for the weary.

The newest addition to indoor tennis around Washington in Georgetown University's Yates Field House, a recently opened all-purpose athletic center that includes a large swimming pool, a diving pool, basketball, various racket sports, running and tennis. For annual membership dues of $320 to $400 (waiting list only), you may play as much as you want on any available facility, without extra fee. The problem is that the tennis courts are surrounded by runners, noise, and downdrafts from the overhead ventilation that sometimes affect the ball's flight. But when snow is four feet deep outdoors, who cares?

A relative rarity these days is the carpeted indoor court, which was more popular when the boom began. One club that still uses carpeting is Linden Hill Racquet Club, which has six courts permanently indoors and covers its three outdoor hard courts with a bubble during the winter. Linden Hill, located off Wisconsin Avenue near the Beltway, is an example of the popularity of indoor tennis even during summer. "We just open the doors and turn on the fans," reports tennis coordinator Una Schonfeld. "The courts are busier than the outdoor ones."

Which brings us to the whole issue of indoor versus outdoor tennis. Unlike the Astrodome idea in baseball, the concept of indoor tennis has for some people taken on a life of its own -- whereas indoor baseball is still something that could only have been invented in Texas. The true indoor tennis fanatic prefers the controlled conditions -- no wind in the hair or sun in the eye -- and constant temperatures of the indoor setting. Una Schonfeld herself is a case in point: "I and many of our members, think we play better indoors. I don't see the ball as well outdoors. My eyes can't take the sun."

Then others want to have it both ways -- playing outdoors in good weather and indoors on the very same courts in winter. This is the concept at Potomac Tennis Club, which covers seven of its 12 well-maintained clay courts for the winter season. Potomac has now become the home club of pro-tennis mogul Donald Dell, his partners Frank Craighill and Lee Fentress, and the favored Washington practice location of their clients Stan Smith and Arthur Ashe. While Potomac is styled very much like an expensive country club for the tennis set ($1,000 initiation fee plus winter and summer court dues), non-members may play indoors during the winter season on much the same basis as any other commercial tennis club ( $60 dues, about $400 for seasonal court time).

One thing that distinguishes indoor tennis from outdoor is that it's organized: The four walls seem to contain the force of anarchy found on the outdoor court. Most indoor clubs provide team structures, doubles tournaments, flight tennis programs (in which your abilities are rated and your partners chosen by a tennis pro) and club ladders (you challenge the player or doubles team on the rung above you).

Tennis parties, combining the social and the athletic, available to member and non-member alike, are getting a big push from overextended clubs trying to sell their unused weekend hours. You can book an entire club for a Saturday evening and stage a round-robin tournament for your friends with a well-stocked bar in the main lounge as the social focus.

Whoever said tennis isn't a contact sport?