Young Gerard Mullen stood on the sidelines, awaiting his turn on the practice field. It was a cool spring night in the Maryland countryside, and the floodlights glaring down on the field at Sandy Spring Friends School bounced off his sweaty teammates-and even glinted off a few slick bald heads.

Mullen is just 16, a sophomore at Rockville High School, but he fits in with the veterans of his Flaps Lacrosse Club. Or, at least, he's starting to.

They still call him John instead of Gerard. "I guess my name's hard to pronounce to these guys," he said. "But anybody who plays Iacrosse is a little bit crazy upstairs."

While Mullen waited to take his place with the third midfield line, he tried to tell what it's like to go against men who are nearly twice his size, twice his age and a dozen times more experienced.

"It's kind of like hitting a brick wall," he said. "The bigger guys knock me around with no problem."

Mullen is 5 feet 7 inches tall, 146 pounds. He joined the Flaps L.C. last year, after just six weeks of instruction, and he earned the team's "Most Determination" award by running into Walt Durigg, a 6-2, 225-pound mid-fielder, three times in a row during a practice.

Such is the spirit of the Flaps Lacrosse Club. The only thing better than a brutal body check or a short-handed goal is the usual one-mile trip up Maryland Rte. 182 to the Olney Ale House for a pitcher or two of beer after practice.

And, such as it is, the Flaps Lacrosse Club-made up of architects, carpenters, students, attorneys, engineers, interpreters, salesmen and the unemployed-has prevailed for more than 15 years. Oh, the name has changed, and the faces, too. But not the spirit.

It started as the Washington Lacrosse Club, then it became the Bowie L.C. and then the Montgomery "B" L.C. and now the Flaps L.C.-but not for long. The sponsorship from Flaps Rickenbacker's, a 19th Street tavern in Northwest Washington, fell through early in the season.

"This is the first time we've tried having a sponsor, other than past support from the Montgomery County Recreation Department," Steven Magdits said. "But apparently there were some misunderstandings on both sides. Now we operate on our dues,"

Which means the $30 registration fee per player will hopefully cover the cost of practice field rentals, referees pay, Iacrosse balls, league membership and the standard keg party which follows each Sunday home game on The Mall next to the Reflecting Pool.

"It (drinking beer) is probably not allowed on the Mall," said defenseman Peter Austin. "But we run a very controlled gathering. At our last home game, three park policemen passed by us and just looked the other way."

Magdits said, "The social aspect of the game is considered as important as the game itself. If you can't beat your opponent on the field, you drink him under the car after the game."

Magdits was president last year of the four-year-old Central Atlantic Lacrosse League-an offshoot of the older, larger U.S. Club Lacrosse Association. The league includes Flaps and 15 other club teams ranging from Philadelphia to Charlottesville, Va. This season he was "elected" general manager of Flaps on a technicality.

"When they were looking for a general manager they asked someone in line to step forward and everyone else took one step back when I wasn't looking," he said.

Aside from the usual postgame keg. there are occasional team parties and a raffle drive with the winner taking home a $100 gift certificate to a liquor store-or cash, if that is, for some strange reason, preferred.

For these social events, Magdits' player registration forms ask each player to name his "wife, girlfriend or mistress." Defenseman Billy Erb filled in: "Sandra/Kristin/Shannon."

The zaniness tones down when Flaps Coach Bill Gibney, a 14-year veteran of club lacrosse who played crease defense at the University of North Carolina, takes command of practice.

The 6-2, 260-pound Gibney is an imposing figure even without stick in hand. He blows his whistle loud and often for as many times as there is a misplayed loose ball or an undisciplined clearing pass.

But for every minute of amateurism there is a moment of grace and spontaneous skill that comes only with experience.

Durigg, 40, has played Iacrosse for 27 years and was on the same Maryland midfield line with present Terp Coach Buddy Beardmore in the early 1960s. Crease attack Chip Markell (Princeton '63) has played Lacrosse for 31 of his 37 years and the 35-year-old Erb (SUNY '65) has 22 sesons under his face mask.

Along with 42-year-old middle Al Griest, the club' senior citizen, they defy the general rule that a Lacrosse player's playing days are limited to four or five years after college.

The Flaps lineup includes a small but representative sample of the bastion of the college game today. Howard Zeskind (14 goals, 12 assists), a 1967 graduate and All-America at Brown, joins Markell and Miche Booz (21 goals) on the front line, while workhorse Mike Robinson (18 goals), a 1975 Maryland grad, sparks the midfield. Goalie Jeff Singer (MIT '77) anchors the defense.

"Against any of the major college teams, we might get wiped off the field," said Gibney, who took over as coach this Spring after backup goalie Jon Weston directed the club to a 6-0 division record in 1978. "Against the smaller schools, we're 50-50. The talent gets better every year."

Robinson drove the middle and scored with just four seconds left last Sunday to give Flaps a 16-15 victory over Riordan's of Annapolis on The Mall.

The victory moved Flaps into a first-place tie with Riordan's in the league's Southern Division. Each team has a 3-1 division record with two games left. Flaps is 5-2 overall.

Flaps took an 11-5 decision from rival Alexandria in a recent contest that exemplified the intensity and respect both teams have for lacrosse.

When Durigg took Alexandria's Scott Liddle out of a play with a retaliated with a questionably flagrant slash. Durigg, much the larger of the two, retaliated no further. Both were penalized one minute.

"I don't get angry," Durigg said. "There's a simple rule that says if you don't like it on the field you can always step back across the sideline."

Wim Cassard, at 18 already a starting middie for Flaps, reacts differently during the heat of battle. After himself checking an Alexandria opponent out of bounds, Cassard did not even watch to see where his victim landed and hustled back into action.

"Did he fall down?" Cassard, a Bethesda-chevy Chase High student, asked a teammate. His cohort nodded confirmation. "Good," Cassard added. "Promising youngster strikes again." CAPTION: Picture, Mike Robinson of Flaps Lacrosse Club clears ball ahead of Alexandria's Clint Emerson. By James A. Parcell-The Washington Post