George Martens, an unheralded jockey who grew up in the shadow of Belmont Park, rode today as if he owned the track.

He guided Summing to a victory in a Belmont Stakes that ruined the Triple Crown aspirations of Pleasant Colony, and in the process he upstaged some of the most illustrious members of his profession.

Martens saved ground, took advantage of a slow pace and made his move at the optimal moment, opening a commanding lead as he turned for home. He had taken control of the Belmont before his chief rivals had begun their rallies in earnest, and held off Highland Blade by a neck. Pleasant Colony finished another 1 1/2 lengths behind, in third place.

Summing ran the 1 1/2 miles in 2:29, a mediocre time over a fast track, and paid $17.80, $7.80 and $4. Highland Blade returned $11.40 and $5.60, and Pleasant Colony, who had been favored at 4 to 5, paid $2.80 to show.

"Hey, he's still a good horse," said trainer Johnny (Fat Man) Campo, who had predicted an easy Triple Crown victory for Pleasant Colony. "That's just the way it goes. He's still No. 1 in my book. The pace beat him. It was a terribly run race.

"It's not the horse's fault. If he got beat by 10 lengths, then that's something else. A better horse beat him. He was really trying to run. He wasn't giving up. He wasn't backing off. He wasn't gaining any ground and he wasn't losing any."

While Pleasant Colony had been winning the Kentucky Derby and the Preakness, Summing was still a justifiably obscure racehorse. He had never won a consequential stakes race until Memorial Day, when he captured the Pennsylvania Derby in a performance that could hardly be described as exceptional. But that was the tip-off that this colt was rounding into form.

Four days before the Belmont, he amazed the clockers here with a brilliantly fast mile workout. "I knew then that he was going to win the Belmont," trainer Luis Barrera said. With his horse in the best shape of his life, all Barrera needed was for Martens to ride the perfect race against competition like Jorge Velasquez and Jacinto Vasquez.

Nobody seemed to want the early lead in the Belmont. Most of the riders and most of the crowd of 61,106 probably assumed that the speedster Bold Ego would go to the lead, but his jockey, John Lively, wasn't interested. A half-dozen horses were running in a tightly bunched pack, loping a half mile in 48 2/5 seconds and three-quarters of a mile in a very slow 1:14 1/5. Pleasant Colony wasn't even close to this group, running 11th and last for the first half-mile.

"When the pace is that slow, it's a terrible race," Campo said. "They went so slow in front of him. But you can't make any excuses."

Martens and angled Summing to the rail from the outset and saved ground, but he knew that the rail can be a dangerous place in such heavy traffic; it's easy to get blocked. But as the field raced down the backstretch, the horse who had the lead, directly in front of Summing, drifted a bit wide. Martens saw his chance.

"Mr. Barrera had told me to lay third or fourth," Martens said, "but the pace was so slow, and when that horse went out we went right to the lead." Summing had taken the lead before he had even been asked to run seriously. He was four lengths in front as he turned into the stretch with a quarter-mile to go, and only now were Velasquez, on Pleasant Colony, and Vasquez, on Highland Blade, taking up the chase in earnest.

Highland Blade and Pleasant Colony were going to run the last quarter-mile in about 24 1/5 seconds each, and the ability to run a final quarter-mile in :24 or thereabouts is commonly considered the mark of a champion. But because the pace had been slow and Summing had managed to take such a commanding lead, Highland Blade and Pleasant Colony had been put in a position where they couldn't win even with powerful stretch runs.

Summing covered the last quarter in 25 seconds.

"I have no excuses. He tried hard, but I can't carry him . . . He's still a great horse," Velasquez said. "I rode him the same as I did in the Derby and the Preakness. He fired when I wanted him. But then Highland Blade came up and I knew I was in trouble. When we were going for home and Highland Blade was ahead of me and Summing was ahead of him, I knew we were in trouble."

The performance by Pleasant Colony was vastly inferior to his races in the Derby and Preakness. Some of the other jockeys thought he had been upset by a cameraman in the starting gate; he was balky about loading in the gate. But Velasquez said that was no factor. After the rigors of the Triple Crown campaign, his form may have been tailing off. More likely, subsequent performances will show that running 1 1/2 miles isn't his best game.

"He still won the Derby and still won the Preakness. He wasn't disgraced," said Campo, who had proclaimed his horse's virtue so loudly all spring. "Summing is no great horse. It doesn't look like my horse is great, either. He's a good horse."

But Campo, losing for the first time in four races with Pleasant Colony since he took over as the colt's trainer following the Florida Derby, said the loss would not cancel his plans to party tonight.

"We're gonna celebrate anyway," he said. "He ain't a bad horse."