It was only a matter of time, they all agreed. But still it was a shock to the horse-owning set of Potomac when they heard the news.

Thomas Dowd, master of the Potomac Polo Clib, a bastion of the Leisure class, has sold the club's richly sodded playing field to a man who plans to build yet another subdivision of $200,000 homes in the afflient Montgomery County community.

And - to the everlasting consternation of club members - this man who is seeking to profit from subdividing their cherished plot of rolling pastureland, is himself a club member.

"It's really terrible to sell the land after we've played there for 20 years." said Robert Beer, who joined the club when it was first founded in 1957. "It's a crime and should be depicted as such."

Thoug club members says they are angry and disappointed by Dowd's decision to sell the land - for $2 million - to developer Clarence Gosnell, few said they could blame Dowd. The playing field is located near the junction of Travilah and Glen Roads.

"It's Mr. Dowd's property and he can do with it what he likes," said Robert Beckman, a Washington lawyer and member of the club. "Mr. Dowd at great personal expense has for a number of years provided excellent polo on Potomac."

Dowd said he decided to sell his property because there was so much development springing up around the placed, pastoral setting of the polo club.

"When we bought it (the property), it was in the middle of hunt country. But it's no longer the same. The neighborhood is changing," Dowd said.

Dowd owns a 265-acre farm, of which about 15 acres is used for Polo. Dowd's house and several barns where he kepts his horses are also on the property.

"I was motivated by what I thought was best for myself and my family . . . I don't think I have to justify what I did," he said.

The Potomac Club is one of three Polo clibs in the oldest and largest, and it has an international reputation.

"I think it's a shame that we lose in the middle of Potomac countryside - the recreational opportunity and the polo playing provided," said Paul Pearson, another club member who is also a developer in Anne Arundel County.

Though it was largely unknown except to those who figure in the insulated world of Potomac, the polo club had been dying a silent and natural death for several years. Lyle Gramley, an economic adviser to President Carter, recalled how the club stopped holding coctail parties after the Friday night games a few years ago, and how attendance at the club's dinners and games had been flagging.

The polo players themselves were getting older and it was getting harder to find young people who cared about horsemanship or, if they did, who were interested in the sport of Polo.

The sale of the polo-playing field has also had a ripple effect on the Polomac Hunt Club, long one of the pillars of Potomac's image of aristocratic sophistication. Each March the polo club held point-to-point races on its playing field to raise money - sometimes as much as $11,000 - for the hunt club. The money, according to Harry Semmes, master of the hunt, was used to feed the club's hounds.

But an optimistic Semmes said, "We'll just have to go somewhere else for the race." He said he had been looking into sites around Darnestown, which is about four miles from the Potomac Polo Club.

Without the Potomac field, the polo players will also have to travel farther for a game - a prospect that some don't like. The nearest polo clubs are the Gone Away Farms Club in Poolesville and the Lincoln mall Polo Club in the District.

Joseph Muldoon, a Washington lawyer who formed The Gone Away Farms Club two years ago, sees the end of the Potomac field as a ticket to success for his young club. Though it is a newcomer, Gone Away Farms has already attacted some of the Potomac Club membership.

Muldoon is making plans to increase the number of games and the number of playingfield at his club on River and Hughes Roads.

Dowd insists that polo will still be played on his field for another year before construction of the housesbegins.

Griff Gosnell, the developer who purchased Dowd's property with his polo-playing father, said construction will go on for about seven or eight years. About 60 homes are planned.

"I don't think anyone should write an obituary for the polo club because the grounds are being sold," said long-time member Robert Beckman. "We'll find other places to go . . .so we'll have to drive 20 or 30 mimnutes to get to a polo field."