Michael Spinks carried exactly 200 pounds onto the scales in the lobby of the Riviera Hotel convention center today. A few minutes short of high noon, the undisputed light heavyweight champion of the world worked his way through a crowd of about 300 spilling out the great glass doors and carrying on like a bunch of sick calves on the way to a sale barn, and resolved one of only two mysteries that have intrigued observers of his camp since he set up here more than three weeks ago.

Spinks (27-0) had chosen not to disclose his weight because he said he "figured it was just another thing for Larry Holmes to think about." That question answered, Spinks, who usually fights at 175 pounds, now must show that he can trade blows with Holmes, the boss of the heavyweight division who stands to tie the 49-0 record set by Rocky Marciano exactly 30 years ago Saturday. The fight, scheduled for 15 rounds, will be staged in an 11,000-seat outdoor arena at the Riviera and featured on Home Box Office. The main event should start around 10:15 p.m. (EDT).

"I've thought of everything that could possibly happen," Spinks said the other day. "And I'm totally convinced that I'll win. I want it any way I can get it, no matter how. A knockout, a decision, a cut, I'm going to do it. All it takes is one punch."

Holmes, who weighed in at 221 1/2 pounds, said, "The only way he could beat me is if I went out all night, got drunk and didn't train."

In the history of boxing, a light heavyweight champion never has beaten a heavyweight king in a title bout. Billy Conn tried and failed against Joe Louis. Bob Foster fought both Muhammad Ali and Joe Frazier but couldn't pull it off. And Archie Moore managed to floor Marciano with a wicked uppercut before being hammered to the canvas six times and stopped in the ninth round.

This century has seen nine light heavyweights fail 13 times in their attempts to overcome heavyweight champs. Way back, Ruby Bob Fitzsimmons won the heavyweight crown but did not take the light heavyweight championship until after he'd lost the heavier division title. Philadelphia Jack O'Brien made a gallant play at glory and probably came closer than any other light heavyweight by fighting a 20-round draw with Tommy Burns in 1906. But he later lost to Burns in another 20-rounder.

For what it's worth, Holmes has been quick to recognize Spinks as a champion, although he's fond of calling him a "layman" when it comes to the heavyweight division. "I don't feel a need to come down on Michael and discredit him," Holmes said. "He's a champion and there's no point in discrediting a champion. This is a business, Michael knows that. If someone gets in the way and tries to take our business, we fight like hell to keep it."

The last time Spinks lost was as an amateur in 1975; he went on to win an Olympic gold medal the following year. Spinks looks for his big right hand, which he calls the "Spinks Jinx," to find its mark against Holmes, who he thinks will "try to get rid of me early by overpowering me. I'm not going to take a stand when we come out. I'll go for broke, but only if he corners me. If I have to fight, I'll fight."

Holmes won the World Boxing Council heavyweight title from Ken Norton in 1978. During his seven years as champ, he has appeared to be more hungry for money than love, although as just another muffin in the ranks he was regarded as an affable, happy-go-lucky fellow who worked overtime trying to emulate the styles of Joe Louis and Ali. Holmes will make more than $3 million for his work against Spinks, who will take home about $1 million, $900,000 more than in his last fight.

After a workout this week, Holmes said he was one mad fighter -- "mad because my money ain't in the bank and I usually should done got paid already." He then said he trusted no one.

"That's why I'm gonna go out and do my thing," he said. "I'll get rid of Spinks without hurting him."

The campaign to break Marciano's undefeated record probably will end in late November when Holmes fights a hand-picked pudding on national television. Reports persist that Holmes already has made a list of leading candidates, including Marvis Frazier, Alfonzo Ratliff 3rd and Anders Eklund, the European heavyweight champ and an easy mark.

Holmes did away with Frazier back in November 1983, in less than one round. Ratliff, a big-mouthed cruiserweight from Chicago, said in a Wednesday news conference that he would "hit Bull Benton in the shoulder blades and kill him, dead." Ratliff, who also said "the heavyweight champion has a chance to be killed if he steps into the ring with me," fights Bernard Benton on the Holmes-Spinks undercard.

Whomever Holmes chooses for his last fight, he will no doubt make sure his chances of losing are nil. In the last few months, he has received mail from people who claim he's cheapened history by continuing to fight only for the quick millions he can make as he bids to break Marciano's record. It is true that Holmes' skills have diminished -- his legs resemble those of a dispirited flamingo and the left jab that once carried the potential for devastating impact now stings about as much as a popgun.

He's 35 and vulnerable, the heavyweight boss of the International Boxing Federation, which is not to be confused with the World Boxing Association and the WBC, the sport's other two sanctioning bodies.

Promoter Don King says Holmes is the heavyweight champion of the world "today, tomorrow and forever," but just yesterday, cynics seemed impatient to see Holmes end his rule and vanish in the unexcellent glare of ring lights.

Odds are 4 to 1 against Spinks winning the fight King has dubbed "History: A September to Remember," but as Spinks said, "A little voice came to me last night and said, 'It's your turn, Michael, to change the course of history.' "