Seattle Slew gave the Triple Crown its first undefeated winner today, leading all the way over 1 1/2 miles to capture the Belmont Stakes, unchallenged, by four lenghts.

The victory, his ninth, confirmed the superiority Seattle Slew had shown in the Kentucky Derby and the Preakness. Run Dusty Run and Sanhedrin finished second and third, respectively, today just as they had in the Derby, with Iron Constitution, the Preakness runner-up, fading to fifth place in the field of eight. Seattle Slew is the 10th 3-year-old to sweep the Triple.

An excited crowd of 70,229 - second in size only to the 1971 turnout of 82,694 for Canonero II - gave Seattle Slew a tremendous ovation on his return to the winner's circle, Jean Cruguet, the jockey, waved his bright yellow cap to the fans. It was a special occasion. Never in 300 years of American racing, has a thoroughbred advanced so far so perfectly.

The competition took turns trying to make Seattle Slew accelerate on the lead. Spirit Level, Run Dusty Run, Iron Constitution and Sanhedrin all attempted to apply pressure to the dark brown Bold Reasoning colt, but Cruguet never winced.

"Relaxation was the whole story," the French jockey said later. "I knew if I could make Seattle Slew relax we'd have to no trouble. Spirit Level came over from the outside to make us go over from the outside to make us go a little faster (a half mile in: 48 2/5 than we wanted to early.

"But about the three-eights pole, I gave him his head and asked him to run a little more. He had plenty left. It was all over."

Seattle Slew's time for the 12 furlongs was 2:29 3/5 over what, officially, was labeled a muddy track. The strip actually was wet-fast, as times of earlier races showed. No one will compare Seattle Slew's preformance with that turned in by Secretariat here in 1973, when he scored by 31 lengths in 2:24. But, once again, it was more than adequate, Seattle Slew continuing undefeated and unchallednged at a point in his career where, for instance, Secretariat had lost three times.

Still, there were a few dectractors.

"What a dull race," a rival trainer was heard to mutter as he walked back to the barns. "You needed a calendar to time the damn thing."

Mickey Taylor, a part owner of Seattle Slew, shrugged off criticism.

"The horse won today by as much as he had to, which is our only objective, every time," the lumberman from White Swan, Wash., remarked. "All we want to di is win, not set records. We wanted to have a lot of horse left when this series was over . . . and I think we do."

Billy Turner, the trainer, said Seattle Slew will be rested, probably until the Travers Stakes at Saratoga in August. The Arc de Triomphe in Paris is not in his fall plans, the trainer added. Instead, the 3-year-old champion will be pointed for a series of confrontations with Forego, the 7-year-old gelding that has been Horse of the Year the last three seasons.

Seven horses had come this far in Triple Crown history, only to fail over the long, demanding Belmont distance of 1 1/2 miles. Today's bettors were never worried about such a fate befalling their favorite. They poundded him down to 2 to 5 after opening even money.

Actually, Seattle Slew has been late only twice in his career. The first time occurred in the Derby, when he broke poorly from the gate and had to bull his way into the lead by the clubhouse turn. The second time was today, in the paddock.

"They (track officials) called us and told us what time to leaves the barn," Turner noted, "but htere were so many cars parked all over the back-stretch they had to reroute us from our barn to get to where we were going."

Seattle Slew arrived in the paddock six minutes late, nine minutes before it was time to leave for the track instead of having the customary 15 minutes in which to be saddled and prepared for action.

Thurner probably planned things that way, despite his comment to the contrary.

"The only place we can lose the race is in the paddock," he said, meaning if Seattle Slew broke out in heavy sweat he could have had too much of his energy drained there. That didn't happen.

The huge crowd wagered $6,498,117 on the nine-race card, second to the $6,972,209 total of 1971 when Canonero faded to fourth. Pensive (1944), Tim Tam (1958), Carry Back (1961), Northern Dancer (1964), Kauai King (1966) and Majestic Prince (1969) were the others to win the Derby and Preakness, only to lose the Belmont.

Today's purse value was $181,800, Seattle Slew earning $109,080 and increasing his lifetime total to $717,720.

Cruguet was aboard four winners, including the 40-to-1 Road Princess which rallied from off the pace to capture the $85,800 Mother Goose Stakes for 3-year-old fillies.