It's a hot Sunday afternoon in Potomac. The polo game has just ended. One player, face wet with sweat, is angry.

"I'm not hitting as well as I should," says Lizzy Beer, helmet in hand, a thick braid of black hair flopping down her back.

Beer, 19, a resident of Potomac, is a member of the fastest growing group--women--of an "in"-again sport--polo. Fifty percent of the 240 polo clubs in America did not exist three years ago, according to Buzz Welker, former acting director of the United States Polo Association. Three years ago there were only 11 women members of the 1,500-strong USPA, Welker adds. Today there are about 100.

"Women are just going nuts over it," says Joe Muldoon III, head of the Potomac Polo School. Muldoon says he has received 60 responses to a recent ad for the school and more than half have come from women. "They seem to go after it with a special zeal," he says.

One of the last male bastions, polo holds a special attraction for horsewomen, long confined to the fox hunt or show ring. Lizzy Beer says there is nothing in sport to match the thrill of thundering 30 miles an hour down a field, mallet held high, ready to strike. Although polo is considered rough, even dangerous, women say skill is more important than strength.

"People don't believe it when I say I play polo," says Molly Baldrige, 26-year-old daughter of Commerce Secretary Malcolm Baldrige and an experienced player who captained the women's polo team at Yale. She works for the Agriculture Department in Washington. "They say, 'Huh?' But there are just so many things you can do if you ride."

Not surprisingly, the emergence of women in polo has met with resistance from some male players, especially the older ones. Women were not formally admitted to the USPA until 1972, after several well-known women players complained bitterly. Ten years ago, one woman in California was known to tuck her hair under her helmet and paint on a false mustache to play. Still, acceptance of women in polo is not universal. Stories of men riding off the field when a woman rides on are not uncommon.

"They're not strong enough to play polo," grumbles 65-year-old Marion Smoak. "I don't think they can compete with men. They can go out on a Sunday afternoon and play at a reduced level." Smoak, former ambassador and sometime polo player, says the game was made for men. "I wouldn't go slashing into Lizzy Beer if she were riding beside me," he says. "It is a very tough sport. With that has to go a great macho feeling of masculine satisfaction."

"It's not a macho sport," argues C. Maybe Runberg, female editor and publisher of The Polo Posts, a Florida-based newspaper. "The question is the person's ability to stay on the horse. All the women are excellent riders and have a certain gusto. Beer is rated among the top 30 women players in the country. She has tremendous potential. In polo, it's very simple. You either cut the mustard or you don't."

Says Joseph A. Muldoon Jr., father of Joe Muldoon III and president of the Potomac Polo Club: "I'd sure as hell rather have Lizzy on my team than a lot of men."

Polomania is only part of a trend toward equestrian sports in general. Nowhere is this more evident than in the nation's capital, where almost every Wednesday afternoon Ronald Reagan--an associate member of the United States Polo Association--takes off to go riding. "The Reagan administration is very horsy," says Joseph A. Muldoon Jr. "We're going to try to encourage them to come out."

Nancy Reynolds, Bendix vice president and close friend of Nancy Reagan, has already saddled up and gone horseback riding in Potomac. So has National Security Adviser William P. Clark and Peter Hannaford, a well-connected Republican public relations man and former business partner of White House deputy chief of staff Michael Deaver.

"You know what Reagan says, 'There's nothing as good for the inside of a man--or a woman--as the outside of a horse,' " says Reynolds, who used to play polo in Idaho.

A White House spokesman says the president does not play polo, although the USPA says his $30 associate membership fee is paid up. "If Ronald Reagan played polo, it was a long, long time ago," says Reynolds. "Maybe 40 years ago, he hit a ball."

Polo's golden years in America were the 1920s and 1930s, with such legendary figures as Tommy Hitchcock, Cecil Smith and Jock Whitney. Walt Disney played polo. So did Spencer Tracy and Will Rogers. Its popularity, however, declined during the postwar period. But now polo's reputation as rodeo for the rich is intact again.

"During the 1960s, being successful and rich wasn't 'in,' " says Welker, who manages the Willow Bend Polo and Hunt Club in Plano, Tex. "Then came the end of the '70s. People were becoming more affluent. Being successful wasn't looked down on. Now, being successful is 'in.' "

Joe Muldoon III agrees, saying polo's snob appeal is the attraction for some. It separates the haves from the have-nots. And in times of an economic downturn, the appeal may be even stronger.

"I think that the economy is why. The last time polo was real strong was during the Depression," he says. "That might be one part of it."

There are $160,000 "polo villas" in Palm Beach, polo magazines, polo newspapers, polo lithographs, polo belt buckles, polo movie stars (George Hamilton, Tommy Lee Jones), platinum-haired polo groupies sipping Pimm's Cup on the sidelines. Polo has been featured recently in Harper's Bazaar, Vogue and Town & Country. Jerzy Kozinski is currently working on a screenplay of his steamy polo novel, "Passion Play." Ralph Lauren has made a mega-fortune with polo. Prince Charles has got bruised from it. Polo means Perrier and Piaget, Tony Lama boots and boys with names like Brad, Corky and Chip.

Polo is bowling for the rich. Golf on horseback. The Pinnacle of Prep.

Lizzy Beer is sitting on her bed, leafing through a stack of polo magazines. On the wall is her custom-made polo mallet. On her headboard, a bumper sticker: "It Takes Wooden Balls to Play Polo."

"I got the disease and I got it bad," she says, luminous brown eyes wide with zeal. "Polo is addictive. I vowed as a kid never to play. My father and older brothers played and it looked so rough. Now, I don't know what I'd do without it."

She started playing several years ago, at first just "stick and ball" (hitting the ball alone), then played in a few matches. During one of her first games, she got bumped hard, but didn't cry. Now, she says, the men have accepted her.

Polo players are rated by a handicap system from minus 2 to plus 10. Beer's father, investment mortgage banker Robert A. Beer, is a 2-goal player. One brother is a 2-goal and the other is rated 1. Lizzy Beer, 5 feet 9, 135 pounds, is rated 0, a respectable rating for a young player, male or female. The highest rating achieved so far by an American woman for outdoor polo is a 1-goal.

"When I watched the men play, I said, 'Shoot, I can do that.' I can ride better than the majority of 'em." She laughs easily. "I'm sorta gutsy. I'll do anything."

Once during a match she broke a male polo player's arm. "Men just need to be put in their place. They're not that great. They're fat and they have the biggest egos in the world."

And how do the men react to her? "Some will try to kill you. Others won't touch you. I got hit three times in one game in my butt."

Molly Baldrige agrees. "They're either disdainful or they're too much of a gentleman," says Baldrige, who has had her nose broken twice playing polo. "They're too worried about hurting you." Her remedy? "As soon as you can, you hit as hard as you can."

A freshman at American University, Beer played on five polo fields around the country last year that had never hosted a woman polo player. "I've met so many famous people at polo games," she says. "Merv Griffin, George Plimpton." She met Prince Charles in Florida. "He's a good player," she says, "but he had a fat butt and a fat stomach the last time I saw him." As for Lady Di, Beer wrinkles her nose. "His wife is such a fuddy-duddy. She's trying to stop him from playing polo. He's gonna divorce her. She doesn't even like horses."

Lizzy Beer loves horses. She owns 10 of them. She says good polo ponies cost anywhere from $4,000 to $12,000 and you need at least four for a game, which is divided into six 7 1/2-minute chukkers.

There's only one thing she likes almost as much as horses: polo players. Especially the Argentine ones. "Polo players are sooo good looking, you could scream," she says. But right now, her love life is at a trot. "When I go to Florida I date a lot of polo players," she says. Big sigh. "A lot of them are jerky though."

Beverly Vaughan's makeup is melting.

It's 90 degrees and the 32-year-old Washington economist with the U.S. Trade Representative's Office is hitting a beach ball with a short wooden mallet on the lush field of the Potomac Polo Club. Welcome to Saturday morning polo school.

"To tell you the truth, I never thought I'd get interested in polo," says the diminutive Vaughan, wearing tight breeches and riding boots. She has on frosted peach lipstick. "But once you have tried this, it's just so much fun!" She started riding three years ago, joined the Capitol Hill Equestrian Society and decided to try polo.

There are a dozen students on the field, men in jodhpurs, women in suede chaps, batting beach balls back and forth. Each has paid $10 for the 90-minute lesson, which includes a videotape of each one's performance.

Joe Muldoon III, 26, stands watching. The 6-foot-4 Muldoon--known as "Little Joe"--is Central Casting's idea of a polo instructor. He is tan and looks like Chevy Chase imitating Aly Khan. He wears aviator shades, tight jeans and tall, brown riding boots. His shirt is unbuttoned at the neck. The women giggle and hang on his every word.

"Okay, we'll start with the outside forehand," Muldoon says, straddling the wooden horse in the batting cage, a fenced-in area where players practice their swings. "What we like to have is a very secure seat. It would be pretty hard to hit a golf ball on a frozen lake with roller skates, so we want a real secure seat. The way you do this is to rotate your left leg, kick your heel out and it drives your knee right into the saddle."

After a while, Muldoon takes the students to the outside arena where they mount horses and gallop around the ring. A blond woman stands on the back of a pickup, videotaping each student. Muldoon announces that next week the group will watch the tapes and enjoy a box lunch. From Ridgewell's.

"I came out here last year and saw these people play," says Karah Henry, a 24-year-old legislative assistant to Sen. Mack Mattingly (R-Ga.). "I just bought myself a horse. I'm horse crazy . . . I'm fairly athletic, even though it's a tough sport, and I worry about it being too demanding. But it's a thrill. It's a challenge to myself. I find it helps my job. If I can move a several-hundred-pound horse," she says, "I can tell a senator what to do."

Her freckles shine in the sunlight. "If I can handle this horse, who's really dumb, I can certainly get some legislation through!"

Yes, she says, being horse crazy is expensive. "I bought the horse for several thousand dollars. It costs about $150 a month to board and feed him. I figured, I'm so close to Virginia hunt country, right in the heart of things. While I'm here, I really want to learn how to ride. It's definitely an 'in' sport, with the Reagans in the White House."

But polo enthusiasts say the sport is no more costly than owning a sailboat or joining a country club. They are trying--without much success--to dispel the myth that polo is only for the rich.

"It's wrong to equate money with all of polo," says Little Joe Muldoon. "You can have one horse and play polo. Basically, what we're trying to do is get people to realize it. It's not just snobby aristocrats going out and playing tiddlywinks. It is a sport."

And a status sport at that.

"Oh I'm certain," whispers Karah Henry in confidential tones. "In fact, I was thinking on my way out here how many Ralph Lauren shirts I would see. But as I pointed out to a friend last year, people who really ride don't wear them."