Canadian Equestrian Team member Alexa Bell aboard Glandor Akai and U.S. individual rider Polly Howard on Riviera City combined talents to win last night's $12,500 international open rotating pair relay in the Washington International Horse Show at Capital Centre.

Sixteen pairs took the confusing course, in which the entire set of obstacles must be jumped twice in correct order by one or the other member and one change of rider is mandatory. In case of jumping fault, the other rider must pick up where his teammate left off. Washington rules also call for riders to be dressed in Halloween costume.

Bell and Howard, along with other members of the Canadian team, were dressed as a jazzercise workout team. Their horses jumped eight fences once each without fault in a fast, 42.11-second clocking.

Untold Story with Beezie Patton and Walito, ridden by Barney Ward, were timed at 44.28, clean, and dressed as Bonnie and Clyde. Third place went to Orchidee with Werner Peters and Ramon shown by Ludger Beerbaum, dressed as devils and representing the Federal Republic of Germany.

Bell's winning ride aboard Glandor Akai put the Canadians in the lead in the team competition with 36 points to 32 for West Germany and 31 for the United States.

Forest Country, a 16-year-old bay gelding ridden by Meredith Michaels, 14, of Hidden Hills, Calif., outjumped a field of 27 entries to win the $10,000 Senators Cup, a jumping class for juniors during the afternoon classes.

Runner-up was Westwind, an 8-year-old chestnut gelding ridden by David Ziff, 13, and owned by Charles J. Ziff, the rider's father. Ziff, a millionaire Dallas real estate man who owns Commonwealth Park in Culpeper, Va., which sponsors the Senators Cup.

Jupiter, an aged bay mare ridden by Becky Warner-Jolicoeur for Mountain Shadows Farm of Albuquerque, claimed top honors in a seven-obstacle jumpoff in the $10,000 Ambassadors Cup for amateur-owner jumpers.