The Baltimore Stars might be the defending U.S. Football League champions, but nobody would mistake them for grownups.

To hang out with the Stars is to go spinning back to your college days. You are in constant danger of having your bed short-sheeted, your shoes filled with shaving cream. There is a feeling that at any moment you could become embroiled in a food fight. If you aren't careful, the top could come off the salt shaker and kicking will begin under the dinner table.

It's part of the charm of the Stars, a giggly group of fraternity boys whose average age, equally boyish Coach Jim Mora once calculated, is 24. It's also one explanation for their resounding success in the USFL. They display a back-thumping, whoop and holler sense of exuberance rarely found outside of college.

"They have a lot of fun," Mora said, "and that contributes to their winning. Then they win, and that makes them have even more fun."

The Stars have been to the USFL championship game twice, losing to the now-defunct Michigan Panthers in 1983 but defeating the Arizona Wranglers last season for a two-year record of 35-6, the best in the league. It's the kind of record usually amassed by slick, militaristic NFL teams, not a group of schoolboys.

The Stars' camaraderie is a natural one. Because of the closeness in age of the players and the territorial nature of the USFL draft, many were teammates in college. Quarterback Chuck Fusina and wide receiver Scott Fitzkee were roommates at Penn State. There are eight other Nittany Lions on the roster, along with 11 players from North Carolina, five from Pittsburgh, four from Temple and three from Cincinnati.

The Stars' humor naturally leans toward the sophomoric. Bogus memos with the forged signature of Carl Peterson, the president/general manager, frequently are posted on the bulletin board. Injured players are sent to a physician who turns out to be a gynecologist, rookies are given false addresses where they can pick up free hams. Offensive lineman Irv Eatman, two years ago a No. 1 draft choice out of UCLA, recently shaved his head on a bet.

"There are some sick guys on this team," said Eatman, nicknamed "Dawg" because he barks at practice. "It's always something, it never stops. They'll sit for hours just to think up a good one. It's hard to be in a bad mood around here."

The practical joke has become the most revered art form. Anyone who falls asleep in the training room is liable to have his watch set forward and wake up to an empty room. A glance at his wrist will tell him he missed the team plane.

Rookies are told to put funnels in their belts and quarters on their noses. They then try to drop the quarter in the funnel. Instead, they get a glass of ice water poured down their front.

Running back Allen Harvin, at 5 feet 9 the shortest player on the team, is a target of constant abuse. He once arrived at his locker to find a baby sweater hanging there. On one weathery day in Philadelphia, he emerged from Veterans Stadium to find a group of players standing around and yelling at a Stars helmet that was placed on top of a three-foot snow bank, "Hey Harv! Come on out of there!"

With that cast of characters, there are few restraining influences.

"No one has ever said, 'Hey come on, settle down,' " said Eatman. "Nobody here is that old. We've never been in a serious situation. No matter what it is, someone is looking for a joke."

Fusina, at 27 the second-oldest player on a training camp roster of 72, occasionally takes it on himself to be a sobering factor. But rarely.

"If there's a problem or something, maybe I'll open my mouth," he said. "They're out of college now and they're mature enough to know when to treat it as a business, and if not, they have to learn. If our record was 6-35, it might be a problem, but it's not. We'd be crazy to change now."

Not that there's much chance of it. The Stars take something close to a sense of pride in their slapstick nature. "Sure, we're professionals, but it's not like we're lawyers," Eatman said. "In the NFL, you can't even celebrate any more. That's ridiculous. We're very excitable, but we're dedicated. This is the loosest team I've ever been with. It's the only kind I'd play with."

The Stars get some of their exuberance from managing general partner Myles Tanenbaum, a real estate magnate who has developed some of the largest malls in the country. He also has developed a passion for football. One often-told story has it that on leaving Veterans Stadium on opening day two years ago, flushed with his first victory, he noticed policemen were leading away two fans for unruliness. Tanenbaum raced over and pumped the fans' hands. "Thanks for coming," he said. "Come back often."

"Myles is a real fan, he's very emotional at games," Peterson said. "He's an untypical owner. I think we've spoiled him. He hasn't seen many down faces or had many bad trips on team planes after losses. He's thoroughly enjoying himself."

The Stars balance their predominant silliness with a surprising workhorse mentality. There is little kidding on the practice field.

"Our guys know when it's time to kick off, it's time to go to work," Peterson said.

The work ethic comes partly from Mora. A former Marine lieutenant and understudy to some of the best coaches in the game, he presents a startlingly no-nonsense contrast to his team.

His practices are long, hard-hitting affairs, which might partly explain a training camp injury list of 18 players. After Saturday's 16-10 exhibition loss to the Orlando Renegades, which eight starters missed because of injury, Mora had the team practicing again on Sunday, an offday for most other USFL teams. It came as no surprise to the Stars; they're used to it.

"We established something in our first year, a belief that we work harder than anyone else," Mora said. "I don't know if we really do or not. But maybe because they think they do it gives them confidence."