Affirmed scored by a head over Alydar yesterday to win the 110th running of the Belmont Stakes and sweep the most intense rivalry in the history of the Triple Crown.

Affirmed became the 11th horse to win the Triple Crown, but his place in thoroughbred racing's Hall of Fame was in doubt until the final stride of this 11/2-mile classic.

It was a race in which both great horses and their jockeys displayed courage and determination. And at the wire, Alydar, second, always second in the series, but so close, was taking his best shot.

Affirmed, with jockey Steven Cauthen aboard, was up to the challenge.

"Oh, it was a real horse race, all right," the 18-year-old Cauthen affirmed moments after reaching the winner's circle. "That other horse (Alyard) never dropped off.He had the lead on us, briefly. I guess he took it about the 3/16 pole . . . but there is nothing wrong with my horse.

"Affirmed ought back, and I guess we got back to Alydar about 20 yards from the finish."

Those last 20 yards will never be forgotten by the 65.417 Belmont Park fans and the national television audience treated to an epiceffort by both chestnut colts. A firmed and Cauthen were down on the inside, along the rail, just as they had been from the head of the stretch when Jorge Velasquez, on Alydar, began to crowd the leader.

Valesquez' move placed Cauthen in a precarious position, briefly. The young jockey had tapped Affirmed on the right shoulder four or five times, turning into the lane.

Now, quickly, Alydar was running up alongside, creating very little space between the two arch rivals. Cauthen had to switch his stick, to his left hand, and he did so deftly, never missing a beat as the duel between A&A rapidly developed into replay of their Preakness playlet.

"We got to the front, by maybe a head, at the three-sixteenth pole," Valesquez said. "I thought, maybe, today would be different . . . until very late."

But Cauthen kept whipping Affirmed with his left hand while Valesquez worked on Alydar's right flank. The two were dead even, leaving the 16th pole. "My God, it was there for the winning," Affirmed's trainer Laz Barrera, would say later.

Affirmed, displaying the brilliance and the gameness that are the trademarks of exceptional thoroughbreds, again was equal to the momentous occasion. The Florida-bred son of Exclusive Native poked his nose in front five strides from the finish-and kept inching ahead until under the wire.

Two jumps past the finish line Cauthen finally showed his age. The youngster from Walton, Ky., could not restrain his entinusiasm. The "coolness" for which he is so famous gave way to standing up in the saddle and waving, with the whip in his left hand, to the large crowd.

"I'll forgive him for that," Barrera said. "There's a lot of the little boy in all of us, you know"

Affirmed had defeated Alydar by 11/2 lengths in the Derby and by a neck in the preakness. Their rivalry extends early into their 2-year-old seasons. Affirmed now holds a 7-2 edge, the juvenile divisional title and all the Triple Crown glory.

But Alydar continues to grow in stature in defeat. The Calumet Farm colt pushed. Affirmed through 12 furlongs of fast track yesterday in 2:26 4/5, the third fastest clocking in Belmont Stakes history. Only Secretarist (2:24 in 1973) and Gallant Man (2:26 3/5) in 1957) ran faster.

Yesterday's pace was slow, Affirmed going out in :25, :50, 1:14 and 1:37 2/5. Alydar tracked Affirmed through the early fractions, but once Velasquez asked his horse to go for broke at the top of the stretch, it was a quarter-horse match.

The two horses reached the quarter-mile pole in 2:013/5. They had run around the turn in :11 1/5! Yet their last quarter required only :25 1/5. These obviously were two high-class colts primed for perhaps the best performances of their lives.

The fractional times tell the story: :25, :25, :24, :23 2/5, :25 1/5. From the three-eights pole to the quarter pole they had run impossibly fast . . . yet they finished very strongly, making the 110th Belmont truly a race to remember.

Affirmed paid $2.20 straight and $2.10 to place. There was no show betting.

Darby Creck Road finished third in the field of five, nearly 14 lengths behind Alydar. Judge Advocate and Noon Time Spender brought up the rear.

Affirmed, already a millionaire, earned another $110,580 from the $184,000 purse. He joins Sir Barton, Gallant Fox, Omaha, War Admiral, Whirlaway, Count Fleet, Assault, Citation, Secretariat and Seattle Slew as Triple Crown winners.

It, marked the first time the series was won back-to-back.