At least four times a week, at the end of the workday, Bob Gross, a certified public accountant from Potomac, makes a point of stopping at his club, just off Wisconsin Avenue near the center of Bethesda.

Changing quickly into T-shirt and shorts, Gross proceeds to spend the next hour charging furiously about on a 20-foot by 40-foot enclosed court. Taking turns with an opponent, he slams a small rubber ball against the wall with a racquet at speeds that often exceed 100 mph.

Gross, 44, is one of an estimated 35,000 to 40,000 racquetball players in the Washington area. It is a sport whose popularity has soared in the past decade.

"It's a way to work out for an hour and get up a good sweat, and it's a nice way to get out and meet people." said Gross, who began playing racquetball six years ago at the Jewish Community Center on Montrose Road.

"Compared to tennis you get a lot more out of it in the time allowed. In racquetball, the ball is always in play."

Virtually unheard of 10 years ago, racquetball has become at the beginning of the 1980s what tennis was at the beginning of the 1970s.

"Racquetball is the nation's hottest sport," says Robert Halstenrud, manager of developmental surveys for A. c. Nielsen Co., the Chicago-based marketing research organization.

In 1979 Nielsen survey showed 10.7 million Americans were playing racquetball, a stunning 283 percent increase over the 2.8 million playing the sport in 1976, the first time Nielsen included racquetball in its triennial sports participation survey.

That's still far behind the 33.3 million tennis players, the 35.7 million runners/joggers and the 69.8 million bicyclists Nielsen found in its survey. But in terms of growth percentages, racquetball was right up there at the top although a comparison with joggiing/running is difficult since 1979 was the first year that was included.

It is interesting, observes Halstenrud, that in 1969 the Nielsen survey showed 10.4 million tennis players -- about the same as racquetball has now -- on the eve of the surge in tennis popularity in the early 1970s.

In this area, a Washington Post survey of sports and exercise activities of residents in the city and close-in suburbs showed running/jogging as the leading growth sport.

Of the 40 percent of the survey participants who said they ran or jogged at least some, 32 percent said they were doing it more now than they did two years ago. Largely a phenomenon of the 1970s, running/jogging has attracted a large and zealous following, most of whom take it seriously.

"I think it's here to stay," said Stephen M. Sonnenberg, a Washington psychiatrist and a regular runner.

Historically, observed Sonnenberg, running was part of man's natural activity until the technological advances of the last few centuries.

"I think it's been rediscovered," Sonnenberg said.

"I think some of this marathon business may be a fad and some competitive running may be a fad, but the value of exercise and specifically running is such that people are going to continue to do it."

In the Washington area and nationally, bicycling was the second most popular participation sport. The Post survey showed 41 percent of all area residents did at least some bicycling.

"Bicycling is something you can do right from your own front door. You don't have to keep paying money once you own you own bicycle the way you do for racquetball or bowling," said Larry Black, president of the National Capital Velo Club, an organization of bike racing enthusiasts.

Like many cycling enthusiasts, Black got into biking as a method of transportation, but he soon developed an affinity for racing.

"It's fun to go fast," said Balck, who also operates a bicycle shop. "Most people who are into racing got into it to get more out of their machine. "A bike is more than just a toy."

On Tuesday evenings at Greenbelt Park and on Thursday evenings at the IBM complex in Bethesda, the Velo club holds bike races. This year's average attendance of 190 is just about double that of a year ago, Black said. aIn addition, the club sponsors weekend recreational rides.

As for tennis, it was clearly the growth sport of the early 1970s, but there are some indications now that it may have peaked -- or at least reached a point where growth will be more gradual.

In the Washington area, 32 percent of the participants in The Post survey said they played at least some tennis. But the Nielsen findings of just over 30 million tennis players first came around the middle of the decade, and it has remained fairly constant since.

"There's no question that in the early 1970s there was a real bandwagon on tennis," said Nielsenhs Halstenrud.

"I would say tennis was the stellar sport of the early 1970s, and we still look at it as a growth sport. Racquetball has become the stellar sport, but the kind of increased we've seen certainly aren't going to persist forever."

Throughout the Washington area, in government agencies, commercial club and on college campuses, interest in racquetball has zoomed steadily in the last few years.

At the University of Maryland, for example, 1,109 sutdents have signed up for 373 slots in the physical education department's racquetball program this fall. Tennis had had only 421 requests for 715 positions.

Since 1974 when Robert Bohn, a retired Marine Corps major general, and a group of associates opened the area's first commercial racquetball club in Fairfax County, an additional 17 commercial racquetball clubs have opened here and three more are under construction.