Roberto Duran said muscle cramps in his upper body, arms and stomach caused him to quit tonight's championship fight against Sugar Ray Leonard with 16 seconds left in the eighth round. Fifteen minutes later in his dressing room, Duran said he would never fight again.

"I think at the end of the fifth round I was getting cramps in my stomach," Duran said, "It got worse and worse, weaker and weaker, my arms were weaker. Never before have I had cramps like this."

With a tear settled in the corner of his left eye, the 29-year-old Panamanian, a world champion for eight years and beaten only once in 73 fights said, "Definitely, I'm not fighting anymore."

Duran's corner men, Freddie Brown and Ray Arcel, said they had little idea their man was ready to quit on his feet.

I wish I could tell you what happened," Arcel said. "I do not know. It is a mystery to me as much as it is to you."

Brown: "After the sixth round, Roberto said his right arm hurt some. He didn't say anything else."

Although the three judges' scorecards would show Leonard significantly ahead through seven rounds, Duran said he thought the fight was about even. And though Leonard felt so much in control that he taunted Duran with Ali-like mimicry in the seventh, Duran said he was not impressed with the new welterweight champion.

"I saw him very weak," said Duran, who five months ago won a unanimous 15-round decision over Leonard. "But my body did not allow me to pressure him. He wasn't great."

A lightweight two years ago, now nine pounds heavier at welterweight, Duran said he needed even more weight to be at his best. He said that of Leonard, too.

"He also have to leave the welterweights," Duran said. "You could see he was weak."

Only Duran saw Leonard as weak on this night. Arcel, the 83-year-old trainer who has been in the corner with 19 champions, said, "Leonard fought a beautiful fight. He controlled it from start to finish. Give him credit for it."

So dominant was Leonard, and so bizarre was the fight's ending, that the obvious parallel was the Muhammad Ali-Sonny Liston heavyweight championship fight of 1964.

In that one, the once-fearsome Liston threw all his bombs at the wraith then called Cassius Clay. None landed. All of Clay's snakelick jabs ate at Liston's baleful countenance, turning it woeful.

And after seven rounds, Liston refused to come out for the eighth, saying he had an injured shoulder.

"It's a possibility," Arcel said to a question asking if Leonard's domination eroded so much of Duran's will that he quit before he even sat on his stool.

"Whether it was frustration or that, I simply do not know. I have to talk to the doctor to find out if Roberto was hurting. Something was wrong. I know that. Something."

Nothing in Duran's fierce history as a street urchin and bullying brawler of a champion fighter suggested he would give up in a fight.

"In all the years I've been with this kid (eight)," Arcel said, "I've never seen him throw up his hand like that and walk away from a fight."

With about 20 seconds left in the eighth round, a round in which a tidal wave of Leonard jabs crashed against the dark beach of Duran's face, the man with the "Hands of Stone" flipped his left hand in the air. It was a move that seemed condescending, as if to say, "Who needs this aggravation? I'm going home."

"Never, never, never has he done anything like that in his entire life," Arcel said. "I wish I could give you an honest answer as to what happened, but I just don't know."

Leonard's work tonight was much different from the test of macho in Montreal, a brutal stand-still war of the kind at which Duran excels. Leonard tonight was a dancer early and not a stand-in-one-place pug trading bombs. Arcel, who this summer said Leonard would rate with the best fighters ever if he beat Duran, said he saw in Leonard tonight signs of greatness."

"The thing different was that Leonard moved and boxed more," Arcel said. "He was able to frustrate Duran. Duran was not able to throw a good punch at him before Leonard hit him. He was not able to get off as fast as in the first fight."

Try as they might, neither Arcel nor Brown could offer an explanation for Duran's cramps, weakening arms or inability to handle a Leonard fighting differently.

"Roberto showed no signs of this in training," Arcel said. "He worked good all the time, he threw punches well, he showed nothing of what you would call an ailment."

Arcel also played down Duran's suggestion that making the 147-pound limit left him weak.

"He was strong at the weight," Arcel said.

At 4 o'clock today, Brown said, Duran ate his normal prefight meal, steak and orange juice.

"No sight of nothing," Brown said.

So what happened?

"He just stopped."