First Time and Tuscaloosa shared top honors in the International Open Jump at the Washington International Horse Show yesterday afternoon, both horses clearing all eight fences for a maximum score of 36 points and both posting a time of 30 seconds for the round at Capital Centre.

First Time waS ridden by Tim Grubb, 23, of the British Equestrian Team, which has turned in a surprisingly strong showing this week. Grubb put the British within 10 points of the leading U.S. team with five international open events remaining at the show. The United States has 53 points, Great Britain 43, Canada 19 and West Germany nine.

Tuscaloosa, no stranger to the winner's circle at the Washington show, displayed the form of last year when the 10-year-old chestnut won two major-jump events. Ridden by Dennis Murphy, a member of the USET who is riding as an independent here, Tuscaloosa went seventh in the field of 24 and established the time to shoot at.

Four of the next five horses managed to go clean but none could equal Tuscaloosa's time until Grubb took First Time over the fences.

Third among the 14 who managed to go clean was Melanie Smith on Neil Eustace's Val De Loire in 30.7 Fourth was Viscount, the USET's 1975 President's Cup winner when showing for France, with a time of 31.1. Eight-tenths of a second behind Viscount was Old English, while West Germany's Golden Gate was sixth.

Murphy, who won Sunday night with Do Right, maintained his lead among the national riders with 23 points. Mike Matz of the United States has scored 20 points through the first seven days and leads the international riders.

Robert Baskowitz's Ruxton won the regular working hunter championship Thursday night by placing second in the division's final stake class and finishing the competition in this blue ribbon event with 17 points. The 6-year-old bay, handled by Charlie Weaver throughout the show, had a one-point margin over Hunting Valley Farm's Gozzi, which had won the title and the Not Always Challenge Trophy in 1975.

The stake class was won by Edward Bulkeley's Boren, ridden by Pat Heuckeroth. Ruxton was second, Gozzi third and Edie Spraunce's Riot Free fourth.