THE TREASURE HOUSES of Britain" is an Anglophile's delight. Exclusive, exquisite and extraordinary, it lives up to the great expectations.

Opening Sunday in the National Gallery's East Building, this sumptuous sampling of the contents of more than 200 British country houses covers 500 years of collecting. Paintings, tapestries, furnishings, porcelain, silver and suits of armor -- the finer things in life number over 700 in this show.

Wear sensible shoes and enjoy.

For manageability, 17 galleries plus a few smaller rooms are divided into four historical sections: "From Castle to Country House, 1485-1714"; "The Grand Tour, 1714- 1770"; "The Gentleman Collector, 1770- 1830"; and "The Romantic Vision, 1830- 1985." Any one of these could hold its own as an exhibition.

The house that the National built, by squaring off the insides of its angular East Wing, is based largely on rooms found in British country houses -- which, as castles, mansions and manors surrounded by large tracts of land, are the antithesis of our homespun definition of a country house.

These country houses have their own art galleries. At the National, a "Jacobean Long Gallery" stretches 90 feet and is lined with Elizabethan portraits. The corridor in turn is echoed in one of its paintings -- a 17th- century portrait of the Countess of Arundel, who sits before such a long gallery, then used both for family portraits and exercise.

In a room dubbed the "Dutch Cabinet," the gallery-within-a-gallery illusion is further advanced. The teal green color of the walls, four of the bronze statuettes on the mantelpiece and even the painting above it are identical to features in "Sir Lawrence Dundas and his Grandson in the Pillar Room," painted in 1769. The Dutch Cabinet was where the gentlemen who took the Grand Tour of the Continent would display the Dutch paintings they brought back as souvenirs. One of them here is a moving Rembrandt, "An Old Woman Reading."

At the Uffizi in Florence, the Grand Tourists saw the Old Masters' works displayed against red walls. That treatment became de rigueur back at the country house, and so it is at the National Gallery, where they copied 18th-century designs for the red wallpaper in some rooms and for the red silk wall-covering in another. And you'll find Titian, Rubens, van Dyke and Velazquez here, as well. Accompanying the Old Masters was the newest in home furnishings, Chippendale.

With its rough stone floor effect, the "Tudor Room" is not to be missed, especially since it's the first room in the show. Scuff past "The Lumley Horseman," c. 1580, the earliest known equestrian statue in English sculpture. The elegant armchairs, the two-handed fighting swords, the portrait of Elizabeth I, the fine miniatures, the ornate pendant called the "Gresley Jewel," all grand. But don't overlook Mary Queen of Scots' gold cross and rosary. She wore it to the gallows.

You'll have your favorites -- and the towering, 18th-century "State Bed" festooned with Chinese silk will be one of them. But high on my list is the "Triumph of the Baroque" room that holds the Countess of Kildare's silver toilet service. A fashionable wedding gift from the groom to the bride, this 28-piece set (comb boxes, perfume flasks, pomade pots and the like) was given in 1709.

Displayed nearby, and big enough to take a bath in, is a silver wine cistern made to hold many wine bottles and flasks. Then, from the country house called Knole, there's silver furniture -- shown here as it is back home, with the silver-framed mirror hung against the tapestry. From the time when French influence on English decorative arts was at its height, the elegant silver table, candlestands and mirror are Louis XIV-era stuff that wasn't melted down.

In addition, this gallery holds garter badges and a garter from the Most Noble Order of same; Grinling Gibbons' masterpiece in woodwork, a carving of dead game and fish that overshadows any mere still life; and the Old Pretender's watch.

It is believed that the Old Pretender, James II's eldest son, when he stayed at Glamis Castle at the time of the 1716 Rebellion, left this silver pocketwatch under his pillow.

Guests do that.

Much of the treasure of British country houses has been preserved by the National Trust. A great number of houses have been transferred to the trust, along with their contents and a suitable endowment. Landowners have often done this, in lieu of paying taxes, and in exchange they and their heirs may continue on there, so long as they open the house to the public in summer.

And now it's time for tea.

THE TREASURE HOUSES OF BRITAIN -- Opening Sunday at the National Gallery of Art's East Wing, through March 16. Admission is by free pass, which can be picked up at the Exhibition Desk on the Ground Level of the East Building. Passes are timed by the half hour and are for use on the day of issue. Hours are 10 to 5 Monday through Saturday and noon to 9 Sunday; Monday through Saturday, the exhibition's open till 7.