Thousand-pound ponies pound the turf, riders straining for that offside back shot. Thwack! Longhandled mallets interlock in a frenzied race down the field. Players shout encouragement to one another.

That's polo.

Two thousand years ago, the kings of Persia played it, poets wrote about it, legends were woven around it; and today polo still has an air of glamor as well as excitement. The word conjures up images of the past, of Gatsby women in flowing dresses and floppy hats, passing lazy afternoons fieldside with proper men in striped blazers and white flannel pants. Of tradition, sportsmanship, horsemanship. That's polo, too.

But polo isn't just long ago an far away - it's here and now, with a British military team in town taking on American competition, this weekend and next week.

And nowadays the spectators who come to the field with car top down and binoculars in hand are likely to be outnumbered by more causal fans for whom the game simply means a tailgate picnic surrounded by lush green countryside, or an afternoon talking to old friends.

And new faces are always welcome, say regulars: You don't have to be a member of the club. "Polo operates on love," says Mrs. Walter Beer of the Washington Polo Association.

Though it essentially remains a rich man's game, polo is drawing a wider public, thanks to colorful exhibitions by the likes of the British Combined Services Polo Team, a fiesty foursome that will be playing this weekend and twice again next week.

Since arriving two weeks ago for a friendly round of matches, the British players have thrilled local crowds with their graceful shots and daring horsemanship. The trick is to "to get the horse to go where you want it to go," says Lt. Hugh Humpfrey of the Scots Dragoon Guards.

But that's only part of the game: Once the horses go where they're supposed to, the two four-rider teams must try to hit a small wooden ball through goal posts at the end of a 300-yard-long field. The game is divided into six periods or "chuckles," usually 7 1/2 minutes long, with four-minute intervals between. There are not time-outs to trade in broken mallets, or swap a tired pony for a fresh one.

The ponies themselves are not ponies at all, as they were in the early days of polo; the term is more a designation than a breed. The British team uses borrowed horses. Although a reasonably good horseman can adjust to a new pony, the game becomes more difficult when team members have to adapt to a strange horse, a new climate and different playing grounds.

But the British team, drawn from the ranks of active servicemen who enjoy the sport, has coped handily with such handicaps on the Eastern Circuit. Last Saturday, Brig. Gen. Arthur Douglas Nugent (17/21st Lancers), Maj. Sean Mahony (Scots Dragoon Guards), Capt. Michael Vickery (14/20th Hussars), Lt. Michael Irwin (Royal Marines) and Humpfrey galloped past the Brandy wine Polo Club in Pennsylvania, 8-7.

"Once you play polo, you don't want to play anything else," says Douglas-Nugent, a veteran with 24 seasons under his boots.

The yearly exchange of British and American players began in the early 1970s, when a British army officer stationed in Washington yearned to scrimmage with other lovers of the game. This year it's America's turn to play host.

Although polo appears dangerous, the general says, he has had only once accident. During a game in Kenya, a horse fell on him: he escaped with a broken leg.

The general says the game also requires a combination of coordination and good horsemanship, an eye for the ball, aggressiveness coupled with sportsmanship, the ability to ride well, a sense of anticipation and physical fitness.

Lt. Hugh Humpfrey, the youngest member of the British team, has played polo for five years and says it absorbs one's complete attention, combining the mental and the physical.

It also provides an opportunity for players to meet people all over the world who consider the sport an important part of their lives.

Polo began about 2,000 years ago, and there are references to the game in Persia, Japan and Tibet. It was then transported to India, where the British chanced upon it, saw it as excellent training for cavalry officers and began playing. The 10th Hudsars made it popular in England, and clubs such as Hurlinghand and Ranelagh soon became famous.

The game came to America in the 1870s, when U.S. cavalry regiments adopted it, and it was made part of the horsemanship instruction for cadets at West Point. Even Teddy Roosevelt, in a letter to Senator Henry Cabot Lodge, expressed his love of the sport.

Both here and in Britain, the game's future looks bright, withmany new teams being formed and a number of new players joining and entering into competition. And with that air a genteel past.