Every afternoon at dusk, a yellow Ford station wagon arrives at Johnny Tocco's Ringside Gym on the corner of Charleston and Main streets, and parks near a cinder block building with pictures of giant cockroaches painted on the walls.

Marvelous Marvin Hagler emerges from the car wearing sunglasses and a baseball cap with "WAR" printed on the crown. The middleweight champion -- accompanied only by his managers, Pat and Goody Petronelli, and his sparring partners, Jerry Holly and Bobby (Boogaloo) Watts -- hurries past a throng of television cameramen and enters the gym. In the oppressive desert heat, the place smells like Sunday brunch at a hog farm.

The marvelous one, mindful of his working class roots, has chosen to train in this Spartan, quasi-industrial setting because, "It reminds me of the place I started from," he says. "People want to know why I train here, why I need this type of atmosphere. I'll tell you why: this is my kind of place. It helps me keep up my attitude and my feet on the ground. Tommy Hearns likes being out there putting on a circus, showboating for everybody. I'm out here to do a job, where I can pay close attention to what I have to do."

Hearns, the World Boxing Council junior middleweight champion, will challenge Hagler for his undisputed title on Monday night before more than 15,000 in an outdoor arena at Caesars Palace. As if to display what he once called a "gigantic amount of confidence," Hearns has set up camp in an air-conditioned ballroom with mushroom-like chandeliers blooming on the ceiling. Hundreds of fans, tired of dunking coins into video poker machines, turn out for the workouts each afternoon and applaud the Detroit fighter's "circus maximus."

On Wednesday, for example, Hearns announced that it was "officially kids' day" and called seven precocious thistleweights into the ring and led them through a clumsy round of exercises. Thursday, all those present learned, was "officially ladies' day," and Hearns made room for seven young women who performed jumping jacks to echoing blasts of reggae funk.

Hearns has shut down the rigorous training cycle that marked his days at the Eden Roc in Miami Beach, where he endured about 130 rounds of sparring with game opponents such as Milton McCrory, the WBC welterweight champ, and Don Lee, a top middleweight challenger. Now that he's relocated here, Hearns has slowed his running schedule and quit sparring "because he's been hurting too many guys," Emanuel Steward, his manager and trainer, said the other day.

"He kept telling me he was holding back and not getting the volume of punches he wanted. We lost three guys in Miami. One of 'em went down with broken ribs and the rest said they couldn't stand it. Tommy thought it was time we put an end to all that."

Hearns, who appears to be in excellent condition, said it was not out of the ordinary to quit sparring six days before a bout, but offered no reasons for cutting his workouts down to hand and foot speed drills and a few minutes on the heavy bag. Each session lasts about 40 minutes and ends with Hearns grabbing a microphone and thanking his adoring public for coming out. He then slips into a handsome, terry cloth bathrobe and autographs pocket-sized pictures of himself.

"You always get criticized for something," he said. "But in this fight, nobody can say I wasn't prepared. The hard training's over. It's important that I stay loose and relax . . . When you overtrain, it's like you didn't train at all."

At 30, Hagler is four years older than Hearns and a veteran of 64 professional fights. He has kept up his intense, uncompromising routine and seems to grow more bald and dangerous with each passing moment. At Tocco's Gym on Wednesday, he went four rounds with Holly and Watts, one of only two men ever to beat him, and that was in January 1976. He did the same on Thursday, encouraging his weary mates to "hit me, come on, hit me," then slapping them silly with both hands.

When someone asked him to please talk about "the monster within," Hagler, obviously thinking about Hearns' 3 1/2-inch height advantage, said, "I don't care how tall Tommy is. When I get in the ring, I'm going to be a giant."

The interrogator, apparently not impressed by the reply, said again, "But what about the monster, Marvin? Tell me about the monster."

Hagler said, "When I get in that ring, the monster will come out. It'll take over, it blanks everything out. It's my protector."

Again, the man said, "But what is it that overcomes you? This monster . . . "

Hagler growled, "You and that monster. You want that monster? Just be prepared for that monster. It's come out . . . And don't ask me that again. You hear me? Just don't do it."

It is so very sporting to see how much Hearns and Hagler dislike each other. Meanness, in this context, becomes them. And one cannot help wondering how they managed not to uncork a right hand or two during their recent 12-day, 21-city promotional tour. At a news conference today in a cavernous hall, Hearns called the champion a "midget," only to be called a "freak" a few anxious moments later.

"I didn't come here for bull-jiving shucking," Hagler said. "I came to destroy Thomas Hearns."

"Marvin's like an old car," the challenger retaliated. "After so many miles you just have to dump it."