The Washington International Horse Show opened Sunday evening with white tie and tails, evening gowns atwinkle with diamonds, flag raising pomp and ceremony welcoming international equestrian teams and extra-dry martinis flowing in the $500 box seats.

That remains largely the public perception of horse shows: a posh diversion for the "horsery set," where all that glitters is at least 18-karat. But there is more to the prestigious Washington show, which continues through Sunday at Capital Centre, than gala nightly soirees. That is not entertainment solely for those who, in F. Scott Fitzgerald's memorable phrase, "had spent a year in France for no particular reason, and then drifted here and there unrestfully wherever people played polo and were rich together.

"That is the sort of image that is changing, and must change if the sport is to survive, because there are many of us interested in horses who are not in the upper strata of society," suggested Janet Carter, who operates Chartwell Farm in Potomac, schooling horses and giving lessons primarily to junior riders.

"This weekend, when they have the children's classes, it's the cutest thing you ever saw - all these fat little gray ponies and kids with pigtails. It's darling. There are probably thousands of kids who would love to spend a day at the horse show, and the're not all millionaires' kids.

"I don't think you'll ever get the public passionately interested in showing hunters and that ki nd of thing, because there's a lot of society and fox-hunting tradition involved in that, which the average person simply can't identify with. But show jumping is a whole different kettle of fish" - she undobtedlty meant a whole different pack of horses - "and that's an exciting sport, with excitement and danger. If can appeal to anyone who loves sports, or animals."

Yes, the horse show is more than those tuxedoes and cultured pearls, escargots and champagne in the Capital Club, hoity-toity carriage parades and a gentlemanly ringmaster (scarlet coat, gold top hat) sounding "the call" with a 5-foot-long silver hunting horn.

There is homemade fudge (20 varieties) and popcorn as well as chocolate mousse, and inexpensive Indian jewelry beside the antique tables and $3,500 sculptures on sale in the boutiques in the concourse.

For every $250,000, Olympic-bound jumper who came to Landover from a Gatsbyeque estate in a palatial stable-on-wheels, there is the chestnut-coated pride of a one-horse family who arrived in a modest van pulled by an overworked station wagon.

This show is green working hunters whose owners paid a $250 entry fee and $60 for a stall in the hope of earning perhaps $300 and a more richly valued blue ribbon. It is ponies, the U.S. Parks Police performing their mounted "musical ride," and the Budweiser Clydesdales and Hitch struting their stuff this weekend.

It is a smorgasbord of live horse flesh, presented in vivid color, but most of all it is the international show jumping: intense competition between some of the best jumpers in the world, ridden by four-man national equestrian teams from Belgium, Ireland , Canada and the United States, individual international riders from New Zealand and Puerto Rico, plus 18 of the finezt "civilian" national riders. The purses total $38,000.

They compete over courses, usually made up of between eight and 12 jumps, that have been designed to present problems of varying difficulty to horse and rider.

The wooden fences usually range in height from 4 feet 3 inches to 5 feet 6 inches and in "spread" from 4 to 6 feet. There are three varieties: "Vertical" (straight up and down), "Oxer" (one jump consisting of two elements) and "Triple Bar" (three elements). Fences less than 39 feet apart are considered as a "combination."

Unlike hunters, which are most often judged subjectively on performance over obstacles that simulate those found in the hunting field, jumpers are scored objectively, only according to "faults" and time.

In jumping, there is no penalty for "rubbing" a fence, only for knocking it down or evading it. Each "knockdown" counts four faults. Three faults are assessed for the first disobedience (refusal to jump the fence), six for the second. A third refusal constitutes elimination.

A fall by horse or rider costs eight faults. Additionally, one-quarter fault is assessed for each second the horse takes over the time limit stipulated by the designer for completion of the couse.

Horses tied after the first round compete in a "jumpoff" over a shortened course. Although the rule book is thick, and loaded with subleties, the basic principle of scoring is simple: the winner is the horse that incurs the fewest faults; if faults are equal, the fastest time wins.

The international open jumping is the blood and guts of the show, the division that elevates it from interest only among participants and friends to a spectator attraction worthy of a vast arena.

Washington is the first of the three major shows that make up the internationally prominent North American Indoor Circuit. Madison Square Garden is next week, Toronto the week after. But only in Washington do the international team members compete against "civilians" in open events as well as aganst each other in tonight's Nations Cup.

"Show jumping is rapidly becoming one of the major individual sports for both men and women," insists U.S. rider Bernie Traurig, pointing out that Sunday evening's climactic $20,000 President's Cup class is both part of an expanding U.S. Grand Prix jumping circuit and a lucrative new World Cup series organized internationally by the Federation Equestre Internationale, the sport's worldwide governing body. "We're on the verge of getting a lot of television coverage and corporate sponsorship."

International Management Group, Mark McCormack's sprawling Cleveland-based sports marketing firm, has acquired the TV rights to most U.S. Grand Prix events and is attempting to sell a package to network television. Sponsorship money has been coming in drips, but national TV would bring a waterfall.

Taurig, 33, foresee the greening of his dreams. He is only the second professional to be named to the U.S. Equestrian Team, eligible to ride in all events except the Olympic Games. The first was Rodney Jenkins, also 33, one of 15 riders recently suspended by the American Horse Show Association as result of a controversial horse-drugging investigation.

Most of he horse show riders, even the internationalists, are technica lly amateurs. They are reluctant to talk about the ways they are subsidized in their nomadic life from show to show, and in that way are reminiscent of pre-1968 tennis players. Before open professionalism, their stock answer to queries about how their globetrotting was financed was, "I have a paper route."

The international riders are "invited," their travel and living expenses and the shipping of their horses paid for equally by the Washington, New York and Toronto shows. The Irish entourage is typical: four riders, four grooms, one captain, nine horses. Makeup of the squad varies among the 20 or so shows in which the Irish team competes each year.

Of the team here, three riders and chef D'equipe (captain) Ned Campion are career military men, assigned to the army equitation school. Their horses are owned by Ireland's Department of Defense. When individual shows do not pay their way, the Irish Horse Board provides subsidies.

"Export of horses is a big industry in Ireland, a significant part of our gross national product, and showing internationally is considered and advertisement for Irish horse," explained Campion.

"Back home, show jumping is a big sport, and growing. In popularity, it is only behind hurling, Gaelic football and horse racing. Kids have ponies and show them. Hunting is very popular in Ireland. It's not an elitist thing.

"The Army boys on our equestrian team are probably all from a middles-class background. The vast majority of the people in Irish show jumping are. The were brought up with horses out in the country, on farms, on farms, and have ridden from young age."

Bernie Traurig grew up in Syosset, L.I., the son of vice president of a chain of retail stores. He statted riding at age 11, after his mother got a horse. "I took some lessons, began competing, and pretty soon latched on to her horse and did well in junior competition on Long Island," he said. "Instead of going to college, I went and worked with the U.S. Equestrian Team in the 3-Day Event in 1963."

He is now a professional horseman working out of Hatland, Wis., with Amy Brumder, the leading national amateur rider. He hopes that in the near future, he and many of his colleagues can be recognized honestly, above the the table, for what they are: talented, dedicated professional athletes who work daily from dawn, schooling horses, to past midnight during the indoor show season.

There can be no quesion of Traurig's athletic skill when he dons his custom-tailored riding jacket, stretch britches, boots and velvet hunting cap, and climbs into the saddle, whip in hand. Taking big, sleek and graceful horses over warmup jumps in the terrifyingly small indoor schooling area, and then out into the ring and over the course, requires extraordinary strength, coordination, judgement, courage and good old-fashioned horse sense.