The teen-agers started arriving at 8 on Saturday night, their cars kicking up tge dyst as they drove down the narrow road called Sycamore Landing, off River Road near Seneca State Park. By 9:30 about 350 of them were jammed among their cars in a field in McKee Beshers Wildlife Management Area, a state-owned hunting and bird watching preserve.

Everything was ready: a field to gather in, music from portable tape-decks, some woods to slip into when things got hot and heavy, four kegs of beer in the back of a car and no sign of the cops. Not yet, anyway.

It was a field party, a weekly ritual for hundreds of Montgomery County high school students. It wasn't as big as some, but it wasn't bad. Certainly better than the party at Violets Lock on Friday, which police broke up by 9.30.

Photocopied maps start making the rounds at high schools on Thursdays along with promises of beer and good times in an out-of-the-way field.

The teen-agers at the Sycamore Landing bash -- mostly from Thomas S. Wootton High in Rockville -- said the parties are fun and harmless. "It's the only place you can have everyone and not get in anyone's way," Wootton senior John Morabito said, explaining why the teen-agers congregate in fields.

"We can't go anywhere in Rockville. We'd get kicked out," said Greg Hogan, a tall, stocky 17-year-old Wootton senior who walked through the crowd on Saturday night.

The youths stood in groups, drinking beer from plastic and paper cups. One teen-ager walked around drinking from the side of a plastic beer pitcher.

They talked mostly of the coming year -- the senior year for most of them, their first year as the "real party-ers," as 16-year-old Brian Densel put it. They were determined to set the pace.

They talked about the senior-year picnic held the day before at Lake Needwood, near Rockville, and they said one of their number was arrested for being drunk and disorderly. "The first weeks of school are crackdown weeks," Densel said.

And they talked of past field parties that were broken up. One was tear-gassed last year, and another was cleared by police who circled low in two helicopters, shining a spotlight on the crowd and telling the teen-agers to "leave the premises."

Lt. David Frantum of the Glen Echo Park Police substation said police helicopters are sent to field parties several times in the spring and the fall. "If you can get there with a helicopter and light everything up, it tends to put a damper on things," he said.

Park police aviation officer Dwayne Darnell said he was flying toward Violets Lock on Friday, but turned back over Cabin John when he heard the party was dispersing.

Capt. Virgil C. Hottinger of the county police station in Rockville said most of the teen-agers who go to field parties don't mean harm but are breaking several laws. Liquor, traffic, littering and trespassing laws are broken, he said, and there is disorderly conduct, disturbing the peace and driving while intoxicated.

"The big problem is the sheer number of people involved," he said. "Usually it's in a field, but you are talking about a couple of hundred to a couple of thousand people. That blocks roadways. Admission is being charged. No one is seeing that minors aren't being given drink. No one sees to safety. And they usually don't have permission of the person who owns the property."

Densel said the police were certain to come to Sycamore Landing because this was the first day of high-school football season, and the police would want to check the celebrating.

Most of the Wootton football players came, bringing their own keg of beer, but they didn't stay long. "This isn't a real field party," said Billy McDougal, tight end on the Wootton team. "It's a parking lot party. It's not the real thing."

McDougal, who came to the party with his girlfriend, Faith McInteer, was not in high spirits. Wootton had lost 14-7 to Springbrook that afternoon. "We should have won," he said. "We scored in the first half. Then we got overconfident."

But the Stroh's Gang was happy. Seniors Nick, Joe, Ken, Steve and Kevin, who organized the party, said they expected to make about $250 profit. They described themselves as "three wrestlers, a strongman and a fist-fighter" and asked that their last names not be used. They said they want to organize a party every week until it gets too cold.

Nick and Joe sat in the trunk of a large Buick most of the night. Joe wore a Stroh's T-shirt and pumped beer from a keg wedged between his knees. Nick wore white overalls and tried to supervise the tangle of arms reaching for beer.

Ken, Steve and Kevin stood in the road, halting cars when they arrived to demand $2 from each occupant and to hand out plastic cups. "Plenty of cold beer," they said. "Try to park off the road."

This is their first year in business, Joe said, but they are carrying on a tradition begun by the original Stroh's Gang in l979. Kevin said his gang is well-organized and added, "We try to get permission, and we talk to the police" when they arrive at a party.

They didn't have permission to use the McKee Beshers land Saturday, but Steve said later it is often used as a parking lot anyway. He said they hoped to get permission to use some private land on the other side of the road for future parties.

Kevin said the gang would be back Sunday to clean up the mess. On Monday, Steve said they hadn't cleaned up but still planned to.

Joe said the party was a good one, although people weren't drinking enough. A few of the youths were drunk, running through the crowd, stumbling into cars or just standing blank-faced.

"I'm not going with my driver," said one girl. "He's a nut." Another girl complained that the boy she was with passed out. "I couldn't get him into the car," she said. But most of the teen-agers were sober.

The police came just before 10. A squad car from the Germantown station parked at the edge of the crowd, searchlights blazing. Joe stopped pumping beer, jumped from the trunk and slammed the lid closed.

Three officers wandered through the crowd telling the teen-agers to leave. Officer Charles Wright walked around the edge of the field and shone his flashlight through the trees to root out stragglers.

"It's a standard procedure," one youth said as people headed for their cars. But about half the teen-agers stayed where they were. The police stayed along the road, and within 15 minutes the beer was flowing again at the back of the field.

"Only the younger ones are leaving," said senior Gary Fitzwilliam. "The paranoids. The seniors are still here."

One 14-year-old girl who asked that her name not be used said she came to field parties "to get out of the house." She said she had been going to the parties almost every week since she entered Wootton last September. "People like to get drunk," she said. "They just want to have a good time. There's nothing better to do.

"My parents don't want me going to keg parties, but most of them are keg parties," she said. "When I go to field parties, I drink one beer, smoke a pack of cigarettes. If there's hard liquor I will drink that. Once I get pretty drunk, I'll drink beer. Once I came home and my mother started yelling at me. Usually I can handle it, pretend I'm sober. But sometimes I'm not."

Her father came looking for her at a field party behind an abandoned barn on Piney Meetinghouse Road known as the Red Barn, a popular site for the parties last year. "Dad called the cops, and the cops came," she said. "There were three cars. I was scared. All my friends told me my dad was there, and the cops were looking for me. So I just got my boyfriend and left."

The girl's father said the mother of another girl told him where the party was and they went to the Red Barn together.

"My daughter is 14, and the other girl was 17," he said. The 17-year-old's mother was shocked to find her 13-year-old daughter there too, he said, adding,"A lot of parents don't know about it. Kids lie. They tell their parents they're going to a movie. I've had my own daughter do it to me."

But the father said he was concerned most about the danger involved.

A year ago last May, 19-year-old Carole Nixon was killed at a field party in Damascus when she and her fiance were run over by a car. Her fiance, James Scott Clark, suffered a broken leg. The driver of the car, 18-year-old Joseph Lawrence Perkins, was charged with negligent driving and operating a car on private property without the consent of the owner.

Last spring, someone was struck by a car at a Red Barn field party. "The fire-rescue unit was summoned,but the road was so blocked the squad couldn't get in," Hottinger said. Some teen-agers threw rocks and bottles, he said, and windshields were smashed. "We had to have units in from Rockville and Germantown." He said three or four teen-agers were arrested.

Lt. Douglas L. McPhee, also of the Rockville station, said police often run into hostility when trying to break up field parties. "Breaking bottles, rocks come out of the crowd, and once in a while you have assaults on individual officers," he said. But "90 percent of them (teen-agers at a field party) are decent, law-abiding people looking for a good time."

There were no bottles thrown at Saturday night's field party, and no arrests were made. The police sent the youths on their way with good humor. "The sooner you go, the sooner I can get out of here," one officer said. "Let's get these cars out of here."

"Where's the party going to?" one youth asked.

"The party's going to McDonalds" the officer answered.

By ll:30 only about l5 teen-agers were left, waiting for a ride in an old blue fastback car. But there was no sign of the driver or his girlfriend. "They're in the woods," said one of the 15.

"Where's the party moving to?" another asked.

"It's over," said a third.

It was then that an angry mother and father arrived. "I'm looking for my 16-year-old daughter," the mother said. But it had taken her a long time to find the party, and she had come too late.

"I made a lot of phone calls," the mother said. "She was supposed to be at a girlfriend's house." The woman said her family recently moved from Florida, where she had never heard of field parties.

Two of her daughters had come to this party, she said, adding, "One is 18. She's okay." But the other, she said, was in trouble: "I'm going to cripple her when I find her."