To most Washingtonians, the sporting competition that opens at Capital Centre today is no big deal. But by the standards of the horse show world, it's as if the Wimbledon tennis championships were being held in Rock Creek Park or the Boston Marathon were being run through the streets of the District.

The event is the Washington International Horse Show, which is expected to include more than 800 horses and 2,000 riders, trainers, owners and grooms. Among the horses are some of the top jumpers in the country, worth hundreds of thousands of dollars apiece. Among the riders are the future members of the U.S. Olympic equestrian team that will compete next year in Seoul, South Korea.

This year, even more than others, winning in Washington is crucial to the world-class riders aiming for a spot on the U.S. team. Its members will be selected by a committee next year, and the top riders already are working to make an impression. Only the best in the nation accumulate enough wins to enter the Washington jumping events, which follow qualifying shows held around the country during the spring and summer.

Grand Prix jumping, as the highest level of competition is called, attracts more spectators, more corporate sponsorship and more television coverage than any other kind of equine competition except thoroughbred and harness racing. And many of its practitioners, including Middleburg-based Katie Monahan Prudent and veteran rider Rodney Jenkins of Rapidan, Va., live and train in or near the Washington area.

The rules of show jumping are simple: Horse and rider must negotiate a course of obstacles, each 4 to 5 feet high, some also several feet wide. They incur penalties by knocking down rails of the jump, by refusing to jump or by completing the course in a slower time than the minimum allowed. If more than one horse and rider complete the course without fault, they return for a second round over higher fences and race against the clock.

Within those strictures lies a complex relationship between the equine and human athletes, communicated through leg and reins, touch and voice. It can be thrown awry by nerves, or ennui, or just a bad day on the part of either partner. Anyone who has seen a jumper make a 90-degree turn at full gallop and then launch himself over three 5-foot obstacles in a row can see how easy it is for hoof to touch rail.

Horses are natural jumpers, but even those who are good at it often would just as soon go around the fences and walls they encounter in the arena. But the best have the qualities it takes to succeed in any sport: fitness, courage and desire to win.

"Most of the horses here are fairly equal in terms of athletic ability," said Greg Best, at week's end the fourth-ranked show-jumping rider in the country. "The ones who succeed are the ones who want it."

They also are the most expensive. The top jumpers competing at the Washington show are worth $350,000 and up; one sold last year for $1 million. A few are owned by their riders; most belong to stables, to syndicates of investors or to wealthy individuals who buy them as another well-off sports fan might buy a football team.

The most successful riders usually have been at their sport since they were small children ("I started late," said 23-year-old Louis Jacobs. "I was 12."). Many have trained with one of the three or four best teachers in the country, and most now make a full-time living at riding horses.

To preserve Olympic eligibility, it is technically the horse who earns the cash prizes in jumping competitions. A first place can bring $30,000, and the rider often receives a commission in addition to what he or she earns from lessons and product endorsements. A few have careers and moonlight in the show ring, but most find that jumping needs full-time concentration.

Monahan Prudent, in 1986 the top rider on the Grand Prix circuit, knows first-hand how even a slight change of attitude can affect performance. Married late last year, she admits she concentrates a smidgin less now -- and this year she has dropped to 19th in terms of prize money won.

"If you don't have tunnel vision, you slip a little bit," she said. "I'm not thinking of the horses every second of the day the way I was."

The Grand Prix jumpers and their riders are not the only participants in competitions at the International. Besides the lower-ranked jumper divisions, there are hunter classes, where well-behaved mounts jump over smaller fences and are judged subjectively, and cutting-horse competitions, in which the horse must separate one cow from a herd and keep it away without signals from the rider.

The logistics of stabling and feeding all those competitors are staggering. Massive tents sheltering 900 stalls and 90 campers and vans from around the country will cover 25 percent of Capital Centre's parking lot when the show is fully underway (it lasts until next Sunday). The horses and their handlers will create a mini-city, complete with electricity, running water, fast food and practice arenas.

More than 75 truckloads of dirt have been spread atop what only a day ago was the Washington Capitals' arena (this is one of the few times during the season the ice is melted). During their week in Maryland, the four-legged competitors will eat more than 15 tons of grain and 1,000 bales of hay, and will lie on 2,000 bales of straw and 5,000 bags of wood shavings.