JUST 17 miles out from this most transient of towns is a wonderful place called Southern Maryland. Upper Marlboro is its psychic capital, seat of ancient (established 1695) Prince George's County.

Many of the region's great houses will be open to view 10 a.m. to 5:30 p.m. Saturday on the House and Garden Pilgrimage, sponsored by the Forest Garden Club.

The charming, cultivated and cheerfully opionated descendants of a Whig and occasionally Tory aristocracy -- and their neighbors -- raise horses, grow tobacco, ride to the hounds and live, like their ancestors, in beautiful old houses with arresting names like Nihil, His Lordship's Kindness and Mattaponi.

They go to and give marvelous big parties in their big old houses (no one of which is ever attended by less than three generations) and serve such fine foods as oysters and crabmeat, and an engaging dessert named flummery. At Easter, ham is stuffed with watercress. Brooke ham, you learn, is dry-cured, Clageet ham is wet-cured, Bowling stuffed ham is reputed to be the best.

The same help travels from party to party.

Aunts and cousins and uncles obligingly keep an eye on each other's children. Their elders, as natural keepers of treasures and traditions, are revered. They live in their lovely old houses with their lovely old things as long as possible. Hills for eight generations have dwelt at Compton Bassett. Weston, twice set fire by the British and once by a servant girl trying to rid it of the haunts, has been home to the Clagetts for 10.

It is a society that, one suspects, was at its zenith a decade before the divisiveness of the American Revolution. The tobacco economy flourished until the Civil War. The Great Depression took its toll as did the coming of Andrews Air Force Base with its higher paying jobs. The Calverts, last of the lords proprietor of all Maryland, died at Mt. Airy in the early 1900s. Their effects went at public auction. His Lordship's Kindness descended from Darnalls to Sewalls to Daingerfields until the last of the family put it up at the courthouse steps in the 1920s.

The Fairfaxes, up from Virginia after the Civil War, lived at rambling Northhampson until the head of the family, Albert Fairfax, returned to England to claim his title. His brother, Edmund, stayed behind and annually entertained the community at a grand barn dance until Northampton, on the site of the present housing development of the same name, burned down. The British Fairfaxes frequently return to Upper Marlboro to visit their cousins.

In the '30s and '40s, there was a brief but chic incursion from the city. David and Ailsa Mellon Bruce bought and sold His Lordship's Kindness just before their divorce. Ilona Massy with husband, Donald Dawson, moved to Trump's Hill. Cissie Medill Patterson bought badly burnt-out Mt. Airy, where George Washington attended the wedding of Jackie Custis to Nelly Calvert, restored it, re-named it The Dower House and made it her grand domain.

The planter caste of Prince George's, once rich, then so long land-poor, now finds itself land-rich, not always better off. To realize the riches, one must sell the property and with it a way of life. A decade ago, the county had more 18th-century structures standing than Williamsburg; today the percentages are just about even. The marvelous difference is that here people are living the history, as they have for more than 300 years.

People lured by tales of the lovely old houses, regularly approach John M. Walton Jr., coordinator of the history division of the Maryland-National Capital Park and Planning Commission to direct them.

He says, "They have enough money to buy the house and the property, but they're not prepared for the high cost of restoration. That comes as a shock -- that, and the way the taxes go up after a house is restored.

The Prince George's County tax rate used to be a Washington area horror story, but with the support of the citizenry for Operation Trim, taxes have mercifully been reduced. Today, the basic county tax rate on residences is $3.04 per $100 of assessed valuation. It rises to an overall rate of $3.75.47 per $100 of assessed valuation when all four taxing agencies are included: the county, the state of Maryland, the Maryland-National Capital Park and Planning Commission and the Washington Suburban Sanitary Commission.

William B. Amonett, a preservation-minded Prince George's county councilman, says that in 1975 the county's Historical and Cultural Trust (first implemented in 1972 by then Prince George's County Executive William Gullett) requested council legilsation giving a tax break to owners of historic sites. Amonett and other councilmen have various bills in-work to aid the "old-house" homeowner. But nothing, to date, has been passed.

"The minute you put a coat of paint on an old barn, they're around like buzzards to jack up the taxes," says Mrs. Eugene Couser of triumphantly restored Woodstock.

The Jesse J. Smiths paid $80,000 for Content, their large, lovely, historic but dilapidated house and one acre in Upper Marlboro. They already have spent $60,000 on restoration, anticipate another $10,000 for a new roof. Of great aid was an aged carpenter from Calvert County, accustomed to working with precious, derelict houses, known only as "Mr. Adams." Smith, an attorney, is very handy around a house. He oversaw the work step-by-step and did some of the work himself, aided by the couple's teen-age sons.

Damian Hill Yewell and her fiance James Taylor Bransom III have worked since January, renovating a charming 19th-century cottage on the Old Crain Highway out of Upper Marlboro.

"We just went down the road a bit and got some old barn siding (from a friend's barn). It looks beautiful." Lunch hours and between clients, Damien's lawyer-father Thomas B. Yewell nips over to do the fine cabinetry. Damin will stencil the walls and may stencil the floors the young people painted.

"The very hardest part," says Mary Sue Couser, "was finding the right house." Since 1973, the Cousers have worked tirelessly, indoors in the winter, outdoors in the summer, starting at the top of their fine Federal Woodstock and working down . . ."so when we got to the living and dining rooms, we would have more experience." In 1973 they paid $60,000 for their house and five acres. Since then, they may have spent another $40,000, including a herd of sheep.

William Baruch Clagett Addison's family and forbears have had to do with old houses ever since Col. John Addison settled in the 17th century as master of Oxon Hill Manor. Now Bill Addison, as representative in Southers Maryland for Sotheby Parke Bernet International Realty Corporation, deals daily with the delicate balance between buyers and sellers of old and treasured houses. It can be, he notes, wrenching, no matter the financial advantage, for an individual or individuals to give up a property long associated with the family. Sometimes one waits a long, long time for a suitable house.

Addison, who has recently sold two fine old, heavily landed properties in Charles County for more than $1 million each, notes that water-front land is at a premium. The house counts heavily in the cost of a property south of Prince George's. Proximity to Washington makes the acreage sometimes outweight the house. There still is, he says, considerable land in Prince George's County priced $2,500 to $3,500 an acre. Prices spiraled in the 60s, have held pretty steady since.

Intensive questioning could find no more than three interesting old properties presently for sale in Prince George's County. All are in the southern portion. The Horsehead Tavern (a residence) is at Baden. In the Upper Marlboro area, the Blacksmith's House (c. 1830) was renovated by Judge Herold V. Powers.

Charles Hill is being sold by Leslie E. Davies of Charles County for his mother. Its oldest portion is 18th century, the floors are cypress, apparently from a singular stand n Charles County. The property, once owned by Ninan Beall, takes its named from Charles II of England. The medium large house, which is habitable but needs renovation, and 11 acres are priced at $150,000.