It rained and rained and rained, and then it stopped. Fighting with a fury greater than any desert storm, Marvelous Marvin Hagler stopped the rain. Tonight before more than 15,000 at an outdoor arena at Caesars Palace, Hagler successfully defended his undisputed middleweight title against John (The Beast) Mugabi, knocking out the previously undefeated Ugandan at 1:29 of the 11th round of the scheduled 12-round bout.

"The man has a lot of pride," said Hagler (62-2-2 with 52 knockouts). "He wouldn't quit. He doesn't know what it is to lose. I hope he's able to bounce back and get his confidence because the man is one good fighter."

Promoter Bob Arum of Top Rank Inc. said a Hagler-Thomas Hearns rematch is likely sometime in the early fall, probably in September. Hagler knocked out Hearns in the third round last April.

Before the Hagler-Mugabi fight, World Boxing Council super welterweight champion Hearns tonight took away James Shuler's North American Boxing Federation middleweight title with a knockout at 1:13 of the first round.

This was Hagler's 12th successful defense since winning the title almost six years ago. Although he was sluggish in the first two rounds, breathing heavily and at times defenseless against Mugabi's hard shots to his head and body, Hagler came on strong in the middle rounds and boxed his way to the stunning end.

In the sixth and most spectacular round, Hagler at 159 1/2 pounds and Mugabi, 157, squared off on virtually every square inch of the canvas and fought like two angry mastiffs over a soup bone.

In that round, Hagler got the best of Mugabi and seemed on the verge of a knockout, but Mugabi, who had won all of his previous 26 bouts by knockout, was able to avoid the early finish by summoning uncommon strength and courage. When the bell sounded ending the sixth, both fighters stumbled to their corners.

"I thought I had him down and out then," Hagler said. "I knew he was hurt but he kept fighting back . . . , I thought the referee might have stopped it before I came in and did some serious damage."

Through 10 complete rounds, judges Jerry Roth and Dalby Shirley scored it 97-94, Hagler. Dave Moretti had it even closer, 96-95, Hagler.

"I tried to box and move around and go the distance," Mugabi said. "I couldn't."

Mickey Duff, Mugabi's trainer, said his fighter hurt the tip of his right thumb in the sixth round. He also bruised the corner knuckle of his left hand. "Hagler had a wealth of experience over Johnny," Duff said. "John had never been involved in this kind of fight before. He was hurt very badly. He told me in the sixth that he'd hurt himself, but he never mentioned it again the rest of the fight."

Hagler did not come alive until the middle rounds. He fought right-handed early on, but was most effective as a southpaw, using the right-hand lead to set up his left hook. "I tried to confuse him a little," Hagler said. "After I started catching him on the left side, I knew it would be my best offense."

Hagler said Mugabi was best as a counterpuncher. Mugabi would wait for Hagler to throw something before coming back with a shot of his own. By the 10th round, both fighters were spitting blood. Hagler's right eye was almost swollen shut.

"I thought the eye could be bait for me, to make him come at me a certain way and set things up," Hagler said. "But I didn't really put it out there for him. He was counterpunching, he wasn't making a full attack."

Unlike his celebrated April 15, 1985 fight against Hearns, Hagler boxed his way to victory tonight. Against Hearns, he came out charging and throwing everything he had. Against Mugabi, he let the fight play itself out and worked with great patience, waiting for instead of forcing a confrontation.

"I did say this would be an exciting fight," Hagler said. "And it was that, all right. It was rough. I had to get him worn down, bit by bit, and then go at him . . . . My whole body swelled up. But I never had any problem seeing out of my [right] eye."

Mugabi said Hagler's switching from his right to his left side threw him off and made his fight difficult. "He was fighting different ways," Mugabi said. "I tried to stand in there and fight with him. He changed around and that threw me off. He got me down. But I tried my best to take him out. He's a great champion. But I tried my best."

A rematch with Mugabi certainly would be a tough challenge for Hagler, who said, "I'd like to see the man go out and fight as a junior middleweight and leave the middleweights alone."

The cofeature opened with Hearns throwing easy shots to the head and body of Shuler. Hearns, the World Boxing Council super welterweight champion, let go combinations that snuffed the air out of Shuler's lungs. As the two fighters moved to the ropes, Hearns looked relaxed, happy and poised while Shuler's expression showed his utter surprise at being hit so hard. Hearns was pecking away with both hands, doing considerable damage.

"I hit him with a left hook to his body and that brought his hands down and opened him up," said Hearns, the star of the Kronk Gym of Detroit.

The fight ended 1:13 after it started, when Hearns unleashed two right hands that might have crumbled a stone wall. "I hit him with a short right and then a long right from a distance. I had to skip to hit him with that second right hand," Hearns said.

When Shuler collapsed, he let his hands lie at his sides. His corner men rushed to his aid, and his younger brother stood over him crying. After a minute to recollect his senses, Shuler rose and faced defeat. His people moved him to the nearest corner, where he sat on a red stool and looked glumly at his trainer, Eddie Futch.

Shuler earned $250,000 for his work; Hearns, rebuilding his Hit Man persona, made $1 million. When finally he was able to stand and walk, Shuler moved to the center of the ring and congratulated Hearns, who less than a year ago lost similarly to Hagler in the third round. They embraced and it was over, if only for now.

In the featured undercard bout, Gaby Canizales stripped Richard Sandoval of his World Boxing Association bantamweight title with a knockout at 2:47 of the seventh round.

Sandoval, 117 3/4 pounds, never had lost as a professional, winning 29 straight fights with ease. But Canizales (32-2, 116 pounds) fought brilliantly and scored a major upset. The evening's opener after a preliminary bout was scrubbed because heavy rain set back the card an hour, this one ended even more horribly. Down for the third time in the seventh round, Sandoval lay motionless on the canvas, his eyes rolled back in his head. As Canizales danced in his corner, medics wrapped Sandoval in towels, placed him on a stretcher and rushed him to Valley Hospital. At the hospital, Sandoval was reported in stable condition, conscious and conversing.

Neurologist Kazem Fathie said Sandoval was "out for about 14 minutes. He stopped breathing shortly. We were able to start him breathing again by holding his jaws open and administer medicine to decrease swelling of the brain.

"The X-rays were normal. No evidence of massive hemorrhage. Alert and not paralyzed. I'm optimistic he will improve."