Each year since he moved to 406 Seventh St. NW, Ramon Osuna has organized an ambitious show of paintings from the past, starting in 1980 with Spanish colonial portraits and religious scenes from "The Cuzco School," and last year focusing on "The French Neoclassic and Academic Tradition, 1800-1900."

This week he has topped them all with the opening of "The Pleasure of Ruins," two dozen 17th- and 18th-century Italian landscape paintings filled with real and fanciful Roman architectural ruins -- a popular genre among European GrandTourists before the camera came along. It is the most significant exhibition of old master European paintings ever organized by a commercial gallery in Washington.

Two years in the making, "The Pleasure of Ruins" was wisely timed to coincide with the National Gallery's "Treasure Houses of Britain" extravaganza, and offers for sale the sort of now-rare paintings that 18th-century British country house owners were eagerly shipping home from Italy, along with tons of antique and Neoclassical sculpture, ceramics, textiles and marble columns and pedestals, to fill their ever grander Venetian villa-inspired houses.

No fewer than one-third of the rooms in the National Gallery show deal with the impact of the Grand Tour -- and its mecca, Rome -- upon British taste and collecting habits in the late 17th and 18th centuries. Those rooms provide a rather splendid context for the Osuna Gallery show.

Three of the artists in "Treasure Houses of Britain" are also at Osuna. Giovanni Paolo Panini -- who provided five huge canvases for Castle Howard back in the 18th century, two of which are in the National Gallery show -- also produced, with his workshop, the "Capriccio of Classical Ruins" on view here. Claude Lorrain, represented by a small sylvan scene, was the influential French-born artist who legitimized landscape painting in the 17th century and popularized the use of the architectural "caprice," or fake ruin, thereby changing the look not only of landscape painting but also of English landscape architecture itself. And Salvator Rosa -- who also had his impact on proto-Romantic landscape architecture -- is represented by a large painting titled "Landscape With Jacob Watering Laban's Flock," typically dramatic and filled with tortuous, weather-ravaged tree trunks and jagged rocks. It is the single most important work on view at Osuna.

But the biggest surprise is the recently rediscovered 17th-century painter Viviano Codazzi. Called the founder of the Roman school of architectural-view paintings, he demonstrates his skill in five strong works here, the best of them -- a fine, small "Architectural View With a Port Scene" -- bathed in a delicious rosy glow.

That particular painting is unpeopled, but Codazzi's larger works -- like those of his many followers and successors on view here -- are dotted with small figures, some in contemporary dress, others in togas and carrying spears, implying biblical or ancient Roman subjects. It hardly matters. All are casually placed like bit players on a vast, overgrown stage decorated with classical fac,ades, obelisks, columns, urns and fallen pediments. The taste for ruins is the thing, with ancient Rome symbolizing the new ideal of democracy, and ruins suggesting the end of despotism.

So popular was this nostalgic, idyllic type of painting, filled with an implicit yearning for past glories, that even artists who never got to Italy painted in the genre. It is believed that Pierre Antoine Demachy, for instance, who painted the huge "View of the Colosseum Amongst Fantastic Ruins," never left France. And the intriguing small Claude-like "Landscape With an Architectural Caprice," was painted by one Michael Angelo Rooker, who never left England.

Old master paintings such as these -- given good quality and reasonable price -- aren't hard to sell these days. There are museums all over the country ready to receive them, and donors seeking tax-deductions ready to buy them. But they are extremely hard to find, and it is in this department that Osuna deserves congratulations for his persistence. The works here were actively sought out by him and his colleagues, who tracked them down in galleries, auction houses and private collections from Monaco to Uruguay, and from upstate New York to Los Angeles.

Some of the works have come with impeccable pedigrees, notably the Rosa, which once belonged to the great 19th-century London collector Alexander Barker, and before that to a descendant of Prince Alexander Besborodko, right-hand man to Catherine the Great of Russia. Catherine, by the way, acquired part of her great collection from Sir Robert Walpole, owner of a great English country house.

Other paintings turned out to be surprise discoveries, including the Claude Lorrain, and the Rooker, which turned up in a local auction. What is most surprising is the overall quality of the show, which even National Gallery curators agree is, on average, at a level that compares favorably with the paintings in the National Gallery show.

Says National Gallery sculpture curator Douglas Lewis, "There aren't many cities where people can walk out of major museums and into galleries where important paintings and sculpture can be purchased. This is the case in London and in New York, but it doesn't occur in any other American city. The fact that it's beginning to happen in Washington is very interesting indeed. For years, Adams Davidson Gallery which shows comparably important shows of American painting and sculpture was alone in the wilderness."

The show, which is accompanied by an illustrated catalogue, augurs well for the expanding horizons of Washington's more imaginative and aggressive dealers, and suggests the growing possibilities of developing national and international constituencies from this museum-filled city. "The Pleasure of Ruins" will continue through Jan. 5. Hours are 10 to 5, Tuesdays through Saturdays.