Denise Cojuangco rode Kid Gloves through a quick, clean jumpoff round to win the $10,000 Ambassador's Cup amateur-owner jumper classic last night in the Washington International Horse Show at Capital Centre.

Cojuangco, of Manila, had to beat one other flawless jumpoff effort by Cynthia Johnson and Monopoly to win in 31.170 seconds. Johnson, of Summitville, Ohio, and Monopoly cleared all fences in the round, preceding the winner's effort, but was over two seconds too slow. Four other jumpoff competitors could not successfully maneuver the course with the quick turns and accurate jumps required.

Just six of 29 entries finished the first round without knocking down a fence to advance to a timed jumpoff over a shorter course.

In the first round, the ninth fence gave the competitors the most trouble of the 11-fence course, and fell with the slightest tip of the horses' hooves. The course was designed with short distances between fences, requiring quick jumps and longer, slower turns, and extreme accuracy by rider.

In the daytime events, Lucky Draw was first in the over-fences conformation division and fifth in the under-saddle class. Lucky Draw now is in good standing in the contest to be grand champion of the International Hunter Futurity.

The leading hunter, now with 13 points, is owned by Cismont Manor Farm of Keswick, Va., and was ridden by Tommy Serio.

Ritzy Bidder, owned by Debbie A. Layman of Franklin, Tenn., and ridden by Patty J. Stovel, won the blue ribbon and 10 points in the under-saddle class. Stovel also rode the third-place horse in that class, Abracadabra, owned by John C. Kinder of St. Clairsville, Ohio.

The conformation class was judged on the horse's build, substance and suitability to become a hunter. In the under-saddle class the horses are shown at a walk, trot and canter around the ring and judged on performance, manners, way of moving and soundness.

The hunter competition continues today with two more classes before the grand champion is named.

Conversation Piece, owned and ridden by Shelly Stiegler of Green Farms in Connecticut, was the top first-year green hunter in yesterday's segment, placing first and second in two separate classes. Kawahma, owned by Alex Gilbert of Palm Beach Gardens, Fla., and ridden by Stacia Klein, placed first and third in those two divisions.

Meant To Be, owned by Leann Morris and Don Stewart Jr. and ridden by Scott Hofstetter, was the top second-year green hunter.

The International Open jumpers, including teams from the United States, Canada, Great Britain and France and numerous individual entries, will compete tonight for $7,500 in prize money and the Eisenhower Trophy.RESULTS (Horse, Owner, Rider)

Ambassador's Cup, Amateur Owner Jumpers: Kid Gloves, Denise Cojuangco, 0 faults, 31.170 seconds; Monopoly, Cynthia Johnson, 0, 33.4; Skip With Me, Ellen Van Dyke, 7, 42.394; Good Enough, Wendy Chapot, 8, 32.601; Hearsay, Kate Chope, 8, 32.827; Omega, Melanie Pelan, 8, 33.573.

International Hunter Futurity, Over Fences Conformation: 1, Lucky Draw, Cismont Manor Farm, Tommy Serio; 2, Everlero, Showcase Ltd., Terry Brown; 3, Kokett, SBS Farms Inc. and Margarete Hachyan, Louise Serio.

International Hunter Futurity, Under Saddle: 1, Ritzy Bidder, Debbie A. Layman, Patty J. Stovel; 2, Truly Lovable, Mr. and Mrs. Don Stewart and Mr. and Mrs. Ron Gordon, Marianne Steiert; 3, Abracadabra, John C. Kinder, Patty Stovel.

First-Year Green Hunter: 1, Kawahma, Alex Gilbert, Stacia Klein; 2, Conversation Piece, Shelly Stiegler, Shelly Stiegler; 3, Ozzie, William C. Rappolt, Louise Serio.

First-Year Green Hunter: 1, Conversation Piece, Shelly Stiegler, Shelly Stiegler; 2, Cavel D'Or, Consolidated Enterprises, M. Michael Mellor; 3, Kawahma, Alex Gilbert, Stacia Klein.

Second-Year Green Hunter: 1, Meant To Be, Leann Morris/Don Stewart Jr., Scott Hofstetter; 2, Phillips Point, Pam DesLauriers, Marianne Steiert; 3, Memphis, Barbara Trueman/Truesports Inc., Leslie Lenehan.