Their houses are castles, palaces and estates built on a scale unheard of outside of history books and filled with objets d'art sumptuous and important enough to match the rooms.

Passed on by centuries of aristocratic families, the houses are held on to by today's owners despite huge taxes and dismaying tasks like repainting three acres of windows. Some houses still play host to royal guests as they did 400 years ago when Elizabeth I would go visiting, but most guests now are, of necessity, paying tourists.

There are more than 700 such homes in Britain, and 200 of them have lent priceless possessions to the National Gallery's "Treasure Houses of Britain" exhibition, which opens Sunday. Long before the exhibition, a small group of the country house owners banded together to ensure their futures, and they call their houses -- not themselves -- the Magnificent Seven.

Tonight, the Magnificent Seven owners (minus the Earl and Countess of Harewood, owners of Harewood House in Leeds, who could not be in Washington for the opening), all of whom loaned objects to the show, will be feted at a black-tie dinner. And though their visits are not frequent, it was a far easier feat to get, say, the Duke of Marlborough on a plane to Washington than it was to get John Singer Sargent's portrait of the duke's family on board. "I think they just barely got it into the hold of a jumbo jet," the 11th duke chuckles.

Herewith, then, are conversations with the six Magnificent Seven owners who have come to town. Honeymoon at Broadlands

After Charles and Diana spent their honeymoon at Broadlands, its owners noticed an increase in visitors -- and a change.

"We had rather a different crowd of people, almost voyeurs," says Lord Romsey, who lives at Broadlands with his wife and two young children. "You could tell by the speed by which they went through the house. Normally it's about a three-hour tour. These people in 1981 would fly through the house in 45 minutes, just to get to the main bedroom."

Similarly, Lord and Lady Romsey (prounounced RUM-zee) found themselves temporarily the subjects of greater than usual scrutiny. Prince Charles, Romsey's cousin and childhood chum, had been best man at his 1979 wedding to the former Penelope Eastwood. "People naturally were interested," says Lord Romsey, 37. He considers. "Well, maybe it's not natural."

By now, though, tourism at Broadlands has settled down to a normal 150,000 people each spring and summer. Broadlands is the most recently opened (in 1979) of the so-called Magnificent Seven houses, and home of Romsey's grandfather Lord Mountbatten until his assassination by terrorists shortly afterward.

"There's a powerful economic motive" for opening to the public, Lord Romsey says. "Unless you're wealthy, which, candidly, we're not."

Lending National Gallery Director J. Carter Brown their Piranesi urns and Van Dyck portrait will help lure more visitors to Broadlands, the Romseys hope. So will converting a 17th-century farmhouse into a lodge for fishermen who want to cast for trout and salmon in the River Test. So will marketing a line of limited-edition reproductions of Broadlands furniture (an elaborate dining room will retail in the six figures).

Lady Romsey says the family manages "amazingly well" squeezed in a third of the house from April through September, but her husband remembers when their lunch had to be brought across a public area.

"Oh, what have they got for lunch?" he laughingly mimics his curious guests, lifting the covers off imaginary dishes. "Lucky! Roast beef. And he likes it quite underdone. I couldn't eat it like that!" The Battle for Blenheim

Winston Churchill was born in Blenheim Palace.

"It wasn't intended that way," says the 11th Duke of Marlborough, who lives there now and bears a startling blue-eyed resemblance to his cousin and godfather, Winston Churchill. "There are two stories. One is that there was a ball going on and Lady Randolph Churchill felt pains. The other is that she was following a shooting party and she bumped over the ground in her carriage, had the pains and returned to the house."

It doesn't really matter which, if either, is correct. What matters is maintaining Blenheim, the palace that was begun in 1705 for John, the first Duke of Marlborough, as a gift from Queen Anne for Marlborough's victory over the French and Bavarians at Blenheim.

"We consider ourselves the keepers of the national heritage," says the 11th Duke of Marlborough. "We want to keep it for the people." About 380,000 of them pay to tour Blenheim each year.

"We need every cent plowed back into the house and grounds," the duke says. "The Americans are great Churchill fans."

Marlborough didn't inherit Blenheim until his father's death in 1972, which is also when he got the title of Duke of Marlborough. Since then, the duke, 59, and his wife, 42, have devoted themselves to Blenheim and the constant restoration.

The duke worried, he says, before agreeing to loan the gigantic John Singer Sargent portrait of his family for the National Gallery exhibition. "I was persuaded," says the duke. "Mr. Carter Brown thought that was going to be the climax of the whole exhibit."

The family lives in the palace's East Wing -- where they can have 16 overnight guests -- with a butler, housekeeper, chef and nanny for their two children.

"We move out for a few months in the summer to get more revenue," says the duke, who opens up the private quarters for an extra charge, "and there's no privacy. If you want to have lunch outside or put on your bikini -- as my wife does -- you can't do it, unless you want a lot of people looking at you."

"My husband lives, breathes it. The house is you," the duchess says to her husband. "And I think it's become me, too."

The annual maintenance costs total about 1 million pounds a year. "My great ancestor won a famous battle," says the duke, "the Battle of Blenheim. I've got another battle -- for Blenheim." He chuckles. "Mine is taking much longer." The 'Brideshead' Castle

Castle Howard, its domes and lanterns reflected in its lake, is etched in the minds of those who saw it on television in "Brideshead Revisited."

For Simon Howard, "it was the place where we pushed the antique wheelbarrows up and down the halls. We were very sorry when they were restored."

In 1940, the cupola and the lantern of the dome and part of the south front burned down. In 1944, Simon's father, George Howard, later Lord Howard of Henderskelfe, was in a hospital in India when he heard his two older brothers had been killed in action and the trustees of the estate had already sold off the Canaletto paintings. "He sent a wire immediately," recalls his son. " 'Don't sell anything else, I'm coming back to manage the estate.' " George Howard restored the damage and opened the 100-room house to the public in 1952.

"I was 17 when I knew I wanted to live at Castle Howard and run it," says Simon Howard, the third of four brothers and, at 29, the youngest of the Magnificent Seven lenders. He and his wife Annette live in 12 of the East Wing rooms.

The estate employs 120 year-round staffers and another 80 in summer. The house is rented for weddings and parties -- "including the policemen's ball."

"Though we turn over a million dollars a year, if there is a profit, it's burned up in the maintenance," says Howard, who didn't give the annual attendance but said it was up 35 percent after "Brideshead."

Everybody works at meeting the maintenance cost. Not long ago, a visitor, for a better look at a painting, put down her pocketbook. Perry, the golden retriever, picked up the bag in his mouth and brought it dutifully to Annette Howard. Though appreciating Perry's efforts, she returned the bag.

For a more likely source of significant income, Howard is now negotiating the sale of a Bernini (1598-1680) bust of the archbishop of Pisa to the Victoria and Albert Museum. The marble was to have come to the National Gallery, with 15 or so other Howard treasures that did make the trip, but Castle Howard needs the several million dollars the sale will bring. Big Business at Beaulieu

Lord Montagu speaks warmly about Britain's and America's shared heritage -- he even wears a navy tie embroidered with crossed Union Jacks and Stars and Stripes -- but he has an agenda beyond cross-cultural coziness. Although 600,000 tourists annually visit Beaulieu, the Montagu family's home in Hampshire for nearly 450 years, he'd be delighted to welcome a few more.

"It's a big business," he says of the British stately home industry. "Fifty-five million visitors to our historic homes every year and a cash turnover of 100 million pounds that goes a long way towards keeping our houses in order."

By lending two paintings for the National Gallery's exhibit, and persuading other, more reluctant owners to follow suit, Montagu thinks he's reminding Americans of one of Britain's prime tourist attractions.

"The interesting thing is that many of the great houses have been opened since the day they were built," he notes. "The builders were so proud of them that anyone could go through. They were advertised. There were guidebooks. The difference is, it was the servants who got the tips. Now, it's the masters."

Tips -- admission fees -- have kept Beaulieu from crumbling, or becoming a school or hotel, he says. "When I succeeded in 1951 the estate was totally unviable, a loss. I decided to move into a few small rooms and open the house."

Beaulieu (pronounced BYOO-lee) was one of the first of the grand homes to welcome visitors. Lord and Lady Montagu have raised their three children there (a son, 24, works for the BBC; a daughter, 21, is in theater design; the 10-year-old boy is in school).

But Montagu now compares himself to the chairman of a corporation -- he oversees a staff of close to 400 employes in the height of the summer season, supervises shops and catering concerns and hotels on the grounds, bottles and sells wine from the Beaulieu vineyards. He says he has picked up useful ideas from Mount Vernon and from Disney World, which "taught us a lot about litter clearance and managing large groups of people."

"We've won awards for our car-parking design," he adds proudly, pointing out the lots on a drawing of the estate. "We've hidden them in a wood; we can get 2,000 cars out of the way." Similarly, the monorail circling the adjacent museum of historic automobiles is painted a discreet dark green. Yet privacy remains the homeowners' prime problem. "You have a sense of it being lived in," he says of his house. "Fires in the grate, flowers, even children's bicycles. So many French cha teaux are empty mausoleums." At the same time, "there's nothing more tantalizing to a tourist than a door marked 'private.' " Proud Parents of Woburn

The Duke of Bedford, credited with inventing the historic-house business when he opened his private zoo to the public, turned to his wife about 12 years ago and said, "We won't be here at Woburn next year." The prime practitioner of the art of genteel hype not long after left for Monte Carlo and sometimes Sante Fe.

His elder son, the Marquess of Tavistock, "had always presumed that Woburn would miss me entirely and go directly from my father to my son." Lord Tavistock's wife Henrietta dissolved in tears at the news, but cheered up at the thought of calling in her husband's earlier promise. Lord Tavistock says, "When I thought there was no chance of us living at Woburn, I said if we ever did, I'd agree to a third child." Jamie, their third son, was born 9 1/2 months after they moved into Woburn Abbey.

Running Woburn is a diverse business, with tasks ranging from donating 22 imperial deer from the Tavistock herd, the only one in existence, to the species' native China, to putting on a Neil Diamond concert on the grounds, to supervising the staff of 250 in a business that grosses more than $3 million a year. The oil bill alone is $45,000 in a house where Lord Tavistock never finished counting the windows. Some 600,000 visitors come to see the park and the house every year.

Lady Tavistock is a passionate horse fancier and breeds horses at Woburn. "Very often, when I'm going to work in the morning, she's just coming in after sleeping all night in the foaling box," her husband says.

At the core of his 100-odd-room country house is a 12th-century abbey, the gift to the first Earl of Bedford from Henry VIII. The major rooms were built in 1747.

Lord Tavistock, who acquired a taste for cowboy movies while at Harvard, named the group of houses that advertise and lobby jointly "The Magnificent Seven." "We should be eight, including Chatsworth, but the Duke of Devonshire doesn't join anything," he says.

Woburn has 32 pieces in the National Gallery show, including, he says, "millions of dollars of silver stolen in an almost perfect mystery book theft a year or so ago. We got it all back, thanks to a large reward and good police work." Warwick's Mighty Fortress

Warwick Castle was built in the days when your castle was your fortress. There is an armory, a dungeon, a rampart walk from which your private army could patrol the perimeter wall. At the top of the towers, parapets jut out leaving gaps from which you could handily drop boiling pitch and quicklime on your enemies below.

"Warwick was first built in the time of invading Danes, I always tell my wife," says the overseer of the castle, Michael Herbert, smiling across the hotel suite at his Danish-born wife, Vibeke, who works for the clothing company Jaeger.

Though part of the castle dates to 1000, most of it was built in the 1300s. In 1978, the Earl of Warwick sold it to Madame Tussaud's Ltd. And Herbert, chief executive of Madame Tussaud's, found himself chairman of the board of the castle.

Today, says Herbert of the castle on the River Avon in Warwickshire, "when you look out from the castle mound, you're looking at what Britain must have looked like in those days -- you see nothing but forest and trees."

The cost of the view has gone up -- or changed, perhaps. Instead of battling Danish marauders, Herbert faces a yearly maintenance cost of about 200,000 pounds. It's covered by the admission fees of about 680,000 tourists expected this year -- "more visitors than any other stately home of its kind," says Herbert. About 200,000 of those visitors will be Americans.

The items from Warwick included in the National Gallery exhibition are pieces of 16th-century armor (however, by the time they were acquired, the owners were more interested in their decorative value). Warwick's collection of armor, according to Herbert, is surpassed only by the Tower of London's.

The Herberts, who have a home in London and a manor house in Denmark, also have a small apartment in Warwick's spy tower.

"One of the most magical times to be there is when there's snow on the ground with a full moon, and nobody's around," says Herbert.

"That and first thing in the morning, when the peacocks are wandering around the garden," adds Vibeke Herbert.

However, both confirm that staying there at night can be spooky. There is a ghost tower -- its formal name is the Watergate Tower -- but Herbert says he's never seen a ghost.

But they once played host to a young girl whose prize in a contest she'd won was an overnight stay in the bedroom of the ghost tower. Herbert arranged for someone to dress up as a ghost, float into her room and leave an old-fashioned handkerchief on the bed for her to find in the morning. She slept soundly through the whole thing.