Shade is beginning to stretch across the broad green commons of Wheaton Regional Park, gently pushing away the cruel sunlight, when the Marauders adult slow-pitch softball team straggles to the dugout, ready to play.

All day the Marauders, young men and women in their twenties and thirties, have toiled quietly in the air-conditioned comfort of their assorted work places: juggling books, studying plant growth, consulting interest-rate charts for home buyers, waiting on tables, puzzling over computer printouts.

But now, on a near-100-degree evening that almost makes their mitts go limp, they are ready to shout, run, dive in the dirt and generally mellow out. First, Marauders manager Ross Sutton tries to instill a sense of purpose as the team prepares to take on a rival, Barney's Bandits.

"When you make an error, that's an act of God," the lanky real estate agent tells the Marauders, his red Washington Senators cap bobbing in emphasis. "When you make two errors, it's stupidity."

Such scenes are being played out in parks all over the Washington area these nights. Softball, perhaps the most enduring game in the United States, is played here by at least 3,000 adult teams registered with recreation departments in the District and nearby suburbs alone, not to mention countless sandlot pickup games, as well as company and church teams that play independent of other leagues.

There are leagues for dentists, leagues for accountants and for lawyers. Nightly from Capitol Hill, 100 teams representing congressional committees and government offices spring forth from the bureaucracy and fan out over the playing fields of the Mall, scattering tourists and playing out their aggression.

Softball captivates a smorgasboard of humanity, cutting across age, race, sex, income, politics and occupation. It is second in popularity only to swimming, local recreation supervisors say, and it seems to preoccupy more people each year.

The game is "a social outlet for young singles who are looking for something to go to other than swinging singles bars. But the health aspect of playing as an adult is probably the single biggest factor in growth," said Don Law, director of the Adult Softball Council in Fairfax County, a hotbed of softball interest.

Nowhere is it more popular than in Montgomery County, where more adult teams are officially recognized than any other local jurisdiction. Softball continues to be the game of choice, despite an upsurge in the 1970s of interest in soccer, recreation department supervisors say.

At the heavily used softball complexes in Olney, Cabin John and elsewhere in Montgomery, pickup trucks are parked next to Mercedes; octogenarians play near 18-year-olds. There are leagues made up solely of Giant Food Co. employes, of workers from Blue Cross- Blue Shield, Government Employees Insurance Co., the Defense Mapping Agency.

Drive along the Wisconsin Avenue corridor of Bethesda late in the afternoon, said one woman who plays on several softball teams in Montgomery, and you see people wearing team uniforms escaping their offices and heading toward any one of 50 fields the county provides.

"A lot of these people are under pressure all day long," she said. "Come 5 p.m., they throw their computers in the air and leap back into nature. They get really wound up.

" . . . If your schedule allows," she said, "you can play every night a week. You can build your entire summer around it."

The interest, said Montgomery County recreation supervisor Ted Roth, "is awesome. It's as diverse as society itself." There is even a slow-pitch baseball hall of fame for the Washington area, founded in 1980.

The President's Council on Physical Fitness said an estimated 20.9 million Americans over the age of 7 played the game regularly last year, 2.8 million more than the year before. The American Softball Association says it was, conservatively, more like 40 million, one out of six Americans.

And, in fact, much of the growth in Montgomery is in mixed softball, played with varying degrees of seriousness.

Softball is a social activity as much as it is a physical one, supervisor Roth said. It breeds marriage and, occasionally, divorce. "You have husbands and wives playing together, and because of their devotion to the game, arguments may develop out on the field that carry over into the house," he said of the latter result.

Montgomery teams generally pay $200 to $1,000 in fees a season, and sponsors often shell out several hundred dollars for equipment and T-shirts.

"It's not expensive, considering the fun they get out of it," said Arthur Romer, chairman of the board of the E.A. Baker Co., a Takoma Park construction firm that sponsors a team. "We certainly think it keeps us more like a big family in the office. They certainly talk about it all the time."

The game of choice for the Baker team and the Marauders is a variation of softball known as "reverse co-rec," which is designed to level the playing field between the sexes. This mixed recreational game matches teams of five men and five women, with men reversing: right-handers must bat lefty, and lefties must bat right-handed. The theory is that women thus are not forced to spend the evening playing the fences when powerful men step up to bat, a theory that one female player said sometimes "underestimates the quality of the women players."

In a county where by conservative estimate at least 20,000 people play in officially organized softball leagues, the reverse game, where women do the pitching and men the catching, is increasingly popular, Montgomery recreation officials say. The reason is that it is often played for fun, not glory: Losers of reverse co-rec matches typically do not slink off the field with broken hearts.

As the Marauders game this night is about to begin, manager Sutton, 32, tries to psych up the team. Sutton, a former merchant seaman who says he went into deep mourning when the Washington Senators packed up and left, appears to be setting the tone for a serious grudge match. Earlier in the season the opposing Barney's Bandits had knocked a hole in the Marauders' hope for repeating last year's triumph as division champions.

Sutton wanted his players to get through the regulation one-hour 10-minute game as efficiently as possible in the terrible heat.

They were guided, he said, by "an inherent drive to get to the beer and hamburgers afterward."

Marauder Maria Matheos, 32, who teaches commercial courses at Churchill High School, agreed that postgame festivities were a major focus of the team. But at bat, the powerhouse outfielder was all business, her biceps tensed and teeth gritted.

"Ah, jeez," she groaned as she popped a foul ball to left field, prompting a Greek chorus of encouragement and suggestions from her teammates.

Softball, a highly verbal game, is played at Wheaton Regional with a standard but modern litany: "Good stroke," "Straighten it out," "Way to be!", "Make it be there!", and, after a juggled catch: "Way to make it look hard!"

The scene at Wheaton Regional, a sprawling multifield facility east of Georgia Avenue, was pastoral and peaceful in the heat, except for the encouraging cries of the players and an occasional loud noise from the Metro subway construction. Off in the distance, the golden light was fading from the softball diamonds, and the sky began to turn violet-gray as the evening grew less oppressive. A faint breeze arose, and the Marauders were hitting their stride.

In the end they prevailed in a 9-to-1 trouncing of Barney's Bandits, a team of loosely connected friends and coworkers that is named after manager Joe Holland's golden retriever.

The combatants shook hands and the Marauders repaired to a nearby emporium for pitchers of beer and a family size pizza.

It was, everybody agreed, the second half of the evening.