Sweet 'n Low, a 9-year-old gray gelding ridden by Anthony D'Ambrosio, set a world high jump record of 7 feet, 7 1/2 inches in the $15,000 Sunkist international-open jumper Puissance class last night at Capital Centre.

The horse, owned by Mr. and Mrs. Donald Tober of New York City, returned for a fourth round of jumping before a record-setting crowd of 15,628 at the Washington International Horse Show.

"He's just a very talented jumper," said D'Ambrosio, 29. "I had no interest in breaking a record tonight. But he just got better and better as he went along."

McLain, an 8-year-old bay gelding who won the $5,000 Gambler's Choice Stake Thursday night with Barney Ward as rider, was the only other horse of the original dozen to return and attempt that height. The wall had been set at six feet, 6-6 and 7-0 3/4 before reaching the record height.

The toughest part of the class for D'Ambrosio was being readmitted to the course after being disqualified for cutting the first fence in the second round. "It was a Xeroxing problem, not a jumping problem," he admitted, referring to missing instructions on the riders' map in the ring's warm-up area. He said he believed the fence to be optional. Judges approved a return ride, but insisted that the entire course be jumped rather than just the first fence.

But the extra jump didn't seem to bother Sweet 'n Low, who ran until he was a 3-year-old at a Pennsylvania flat track. "I think he could have gone higher tonight," said D'Ambrosio, who has been training the horse for the past two years.

It was the third consecutive year that the high jump record has been broken at the Washington show. Before the fourth round, officials promised an additional $5,000 to the rider who could break the record. Last year, Ward rode Glandor Akai over 7-6 3/4 to break the record set the year before.

In the team competition, no points were added to the United States' 61 points, but Canada managed to reach 26 and West Germany 19 in the evening event.

Ward remains the leading national rider with 21 points, followed by Buddy Brown and Anne Kursinski, who are tied with 14.