Seth Cooper couldn't get over how good the egg rolls were. So he kept popping them into his mouth and soon it was six. Then pop, pop, eight.

Now did he really eat but eight? Not 10 or 11 or 15, maybe?

"I stopped counting," he admitted, reaching again into the warm, greasy pan. "But let's say eight." One bite, two bites, another one gone.

Then he switched gears. "The liver is good, they tell me," he said, eyeballing not only the liver but the table of bubbling fat sausages, baby quiches, trays of butter cookies, platters of cheddar . . .

He moved over for a closer look.

Around him, in Room 2128 of the Rayburn Office Building, members of the New York State Bankers' Association were busy welcoming Fed Chairman Paul Volcker and other significant policy makers to their Capitol Hill get-together. Over there you could see the smiling Citibank people, there were the laughing Chase Manhattan folks and there was somebody from the Senate Banking Committee. An over there was Cooper, eating.

He is what you might call, among the 16,000 on the Hill, a clever congressional staffer. He is tall, attractive and lanky. He makes around $12,000 a year. And he has this amazing diet:

"If it's free," he explained, "it's not fattening."

Almost any night of the week, you can stroll down the grand marble corridors of the House and Senate office buildings and find a party. Or two parties. Or 17 even, whirling on night after night as the truckers or bankers or oilpersons of America set their food and drink before legislators who someday, somehow, might help them at a conference meeting, keep them from harm at a committee hearing, or just plain unsnarl the legal confusion that runs America's capital.

Now, given that there are some 170 trade associations registered to lobby on the Hill, given that there are 535 congressmen, and given that there can be up to 30 parties per day on the House side and up to 75 per week on the Senate side, the enterprising and underpaid Hill staffer can enjoy many advantages in attending dozens and dozens of them. Invited or not.

The first advantage is free food and drink. "All I came to do was snarf down," said one highly anonymous staffer as he stood in line last week for tamales and refried beans at a party in the Longworth cafeteria. "Now I don't have to go grocery shopping."

Free food and drink appear to be the second, third and fourth advantages, too. No one seems much interested in socializing, especially with lobbyists who have a tendency to chew off ears when they see "the office of Cong. So-and-So" written on a name tag.

"Just figures I'd come to soak them for what they're worth," Tad Boggs, a Hill intern, was saying at a hot and humid reception given by the nuclear industry last week. "I'm a no-nukes person myself."

Last week, actually, was a fine week indeed for Capitol Hill partygoing. The nights were warm and lovely, the House was approving the budget, and seemingly every lobbyist in America was in town for a piece of it. By day, the lobbyists stalked the halls of Congress; by night, they spread their bounty.

Just a quick look at three days of the schedule reveals a timber lobby party as well as a birthday fest for Rep. Henry Gonzalez (D-Tex.) on Monday; at least seven parties Tuesday, including one given by the Pennsylvania Association of Intermediate Units (more on what a unit is later); and on Wednesday, a big nuclear blast.

But before taking a closer look at these and other events, some accumulated wisdom from veteran Hill partygoers seems in order. Such as:

Go to anything held by major lobby groups like those in trucking, insurance, oil, steel and electricity. "You should have been at the Edison Electric reception about two weeks ago," one Hill staffer said at a reception last week. "They had scallops, freshly sauteed in front of you, clams on the half shell, veal scaloppine . . ."

Go to anything held in the Rayburn courtyard, Longworth cafeteria or Cannon Caucus Room. Usually, the tables are groaning.

Avoid receptions in the smaller basement rooms of Rayburn as well as public interest groups, which tend to serve only wine and cheese.And sitdown dinners, too. They'll kick you out.

(Ken Doyle, a staffer who was rapidly consuming a chicken leg at a party on Tuesday, was at the same time explaining how he got thrown out of a dinner given by the Sacramento Chamber of Commerce on Monday. "Yeah," he said, carefully spacing his words in between bites of his drumstick, "I went to the door and I looked around and then this woman said, 'Sorry.'")

Avoid people like Jay Treadwell, director of Food Service for the Senate, and Jim Abernathy, the man who runs the $1-million-a-year catering business on the House side. "Some of the young folks," wails Abernathy, "think it's their God-given right to crash." It's a constant problem, he adds.

Check your boss' mail and office bulletin boards for invitations. Often the whole staff will be invited, but if not, you can usually fill out a name tag and meander unnoticed into most large events. Another trick is to call the building security guard and ask him where the receptions are that evening.And sometimes the best technique is to just walk down the hallways, following your nose.

Don't overdo. "When I was an intern," says Ken Rinzler, a staffer for Rep. Robert Roe (D-N.J.), "I used to go to one every night. Jesus, you could have one drink at each party and be an alcoholic for the rest of your life. The idea was to see how many you could crash. But now that I don't have to crash any more, it's not fun. No challenge. I never thought I'd see the day when I didn't want to go."

But for those who still do, three days in May fell gently into springtime twilight, into melon balls with cream dip, into bacon rolls and gin. Monday, May 5:

The big event of the evening is a reception sponsored by the National Forest Products people and the American Paper Institute, honoring Reps. Al Ullman (D-Ore.) and Barber Conable (R-N.Y.). It's in Room 1100 of the Longworth building, it's big, crowded and very easy to wiggle in to. The smell of sizzling beef drifts down the hallways.

At 6:10 p.m., an 18-year-old intern enters. This is what he does until 6:20:

Whooshes through the receiving line. Strolls by the cheese table, taking a quick inventory. Strolls over to the bar and orders a screwdriver. Drink in hand, circles the room, taking a final inventory.

And now, he begins. Sausage roll, then over to the next table. Chicken teriyaki. Another table. Fried brie cheese. Another table. Beef teriyaki. And another table.

The process is rhythmic, almost mechanized. He hits and eats, hits and eats, making a slow circle of the room, sipping his screwdriver in transit. a

"Receptioneering, that's what I call it," he says. "I work for nothing. It's really the only way I can survive."

Downstairs in the Longworth cafeteria, there's a simultaneous Mexican birthday party for Rep. Henry Gonzales. There must be 1,000 people here, most of them standing in line for tamales, beans, rice and beer.

"This party's a good one," says a young guest. He gulps down his Lone Star. Tuesday, May 6:

"This is a good way of getting something into my stomach," says Bob Walsh, a young staffer who is putting quite a few somethings into his stomach. Potato chips, pretzels, Heineken, crackers, tuna fish . . . all the while standing beside the table at the New York bankers reception.

Half an hour later, he appears beside the table of another reception just down the hall. It's for the Pennsylvania Association of Intermediate Units, a unit that turns out to be an educational service agency. There are 29 of them in Pennsylvania, which is of absolutely no interest to Walsh. Neither, as it happens, is the food.

"Same as the other one," he sighs. "I'm full and I'm going home."

Seth Cooper, the congressional egg roll eater who's with him, agrees. "We came to see if there were any clams and oysters on the half shell," he explains. Alas, there aren't. Wednesday, May 7:

Here we are at a party in the Rayburn building given by the American Nuclear Energy Council and other nuclear groups. And look, there's Bob Walsh again.

"Hey, how ya doing?" he says, then vanishes into the crowd three and four deep around the table.

Among the crowd of maybe 500 people is Debbie Roberts, a staffer in the office of Rep. Marty Russo (D-Ill.). She is blond, wears a pink buttondown shirt, a khaki skirt and pearl earrings. She has parked herself in front of the potato chips.

"I came," she says between crunches, "because I thought I would have to work late and I wanted dinner."

So did Gene Carr, a staffer in the office of Rep. Lamar Gudger (D-N.C.). "This is only my second sandwich," he announces proudly over his roast beef on white roll. "And they're pretty good. Dry, though."

Down the hall, the Pennsylvania Rural Electric Association is hosting. This is not as big as the nuclear party, the food's not as involved, and so Ken Rinzler, a staffer who produces an invitation to make clear he's invited, decides to head down the hall to the nuclear folks. He makes clear he's invited there, too.

Along the way, he sees someone he knows coming from the nuclear party.

"Hey, how ya doing?" says Rinzler.

"Pretty, good, how are you?" says the friend, then passes on in the direction of the electric party.

"Jeez," says Rinzler after the friend moves on. "Even the elevator operators crash."