The Washington International Horse Show opens tonight at Capital Centre and will continue through next Sunday with teams from Canada and Italy competing against amateur and professional riders from the United States.

Last year's President's Cup winner and the current World Cup champion, Michael Matz, will lead the U.S. team, riding Jet Run and Honest Tom. Matz, 30, of Plymouth Meeting, Pa., has won four events on the American Grandprix circuit and is the leading money winner with $47,320.

Veteran Rodney Jenkins will make his first appearance with the team at the Washington show since 1975, when he won the International Rider championship. Jenkins, 37, of Orange, Va., is third in the standings and will ride Idle Dice, Coastline and Second Balcony.

Melanie Smith of Stonington, Conn., on Calypso and Belle, and Norman Dello Joio of South Salem, N.Y., on Johnny's Pocket, make up the rest of the U.S. team.

Graziano Mancinelli, Uberto Lupinetti, Fillippo Moyerson and Raimondo d'Inzeo will represent Italy during the eight days of competition, which includes the Prix des Nations class for international teams Tuesday.

Ian Millar, Mark Laskin, Kim Kirton and Shelagh Mecpherson are riding for the Canadian team. Tim Grubb is the only rider scheduled to represent Great Britain.

The Washington show is the only international show in which the team riders compete directly with the national riders, amateur and professional. An international jumping class is scheduled each evening with the puissance (high jump) class Friday and the match race Saturday. The finals of the $25,000 President's Cup Grand Prix will be held on the final evening.

Throughout the week there will be all types of hunter competition for riders of all age groups on horses and ponies. Wednesday night the local foxhunters take over the ring, with classes for field hunters, ladies side saddle and hunt teams.

In one of the special exhibitions, Count Don Alvaro Domecq of Spain will demonstrate bullfighting techniques (against a mechanical bull) aboard one of his Andalusian horses.