A fraction of a second again decided the international-open $1,000 stake at the Washington International Horse Show yesterday afternoon.

Buddy Brown of the United States Equestrian Team and Sandsablaze put it all together for 38.4 seconds at Capital Center over the 10-jump course. The result in the complicated scoring was a 10th-of-a-second margin over Dennis Murphy and Tuscaloosa. Another 10th of a second behind was Rodney Jenkins on Wasted Words.

Monday night's race was also decided by fractions of seconds as Liz Edgar of Great Britain won the speed class on Everest Wallaby, with Norman Dello Joio second on Sky High and Purphy and Tuscaloosa third.

Yesterday's scoring was based on time plus seconds added for each fault. However, the top horses managed to go through the course clean and thus speed was the dominant factor.

Sandsablaze, a veteran horse but not frequently used by the USET, is a handsome jumper, a light chestnut with a full blaze right down to its nose. Its only major win in 1977 was at Chagrin Valley in Ohio as intermediate champion.

It was the first victory of the show by a riding member of the USET. Murphy is showing here as an individual. At 21, Brown is one of the younger members of the team. He was a member of both the 1974 and 1976 Nations Cup winners at the Washington International. Last summer, he was the leading rider at the Devon (Pa.) show.

It was a day of action for the blue-ribbon hunters and some of the show's most prized awards were handed out. Mary Cunningham's Vital Victory, the bay gelding that has won just about everything in its show career, added the Shining Hour Challenge Trophy to the Cunningham Trophy room in Warrenton. Second in the model class was Stocking Stuffer. Mr. & Mrs. August Buseh Jr.'s winner of the trophy in 1975 and the conformation hunter titlist in 1974 and 1975.

Stocking Stuffer came right back in an open class to win the blue and add the Morris Cafritz Memorial Challenge trophy to the Busch collection. Edie Spruance's Riot Free turned in an effortless round to win the Chillum Challenge Trophy. Its closest rival for the regular working hunter tri-color. Gozzi, was a close second and won an under saddle earlier in the day. In its early stage, the division is closely bunched. Dwyer Hill Farms' Another Brother won the Second Marlborough Hunt Club Challenge trophy Monday night and Ruxton won third- and fourth-place ribbons yesterday.