THE GIFT that keeps on giving. For those who keep track of such things, philanthropists Paul and Bunny Mellon have donated 822 works of art to the National Gallery of Art.

In 1985 alone, they gave 186 works, and some of these are showcased in "Gifts to the Nation: Selected Acquisitions from the Collections of Mr. and Mrs. Paul Mellon," which opens Sunday in the Gallery's West Building. This is a fine sampling of some of the best that money can buy, unquestionably beautiful works, works you could live with, by French, British and American masters from the 18th through 20th centuries.

A love of horses first drew Paul Mellon to British sporting pictures in 1931; he was collecting British paintings in earnest by the 1960s. And after their marriage in 1948, the couple began collecting French impressionists, following Bunny Mellon's interests.

One can only imagine how much fun the curators had hanging this show, seeking out resonances among artists and subjects. The first room holds an extraordinary sampling of works on paper, where artists play off one another. Cezanne's self-portrait in charcoal is paired with a similar one by Matisse, both donated last year. Another 1985 gift, Picasso's collage "The Cup of Coffee," demonstrates that artist's incredible eye for composition, in merely glued paper -- and next to this hangs a similar collage by his co-cubist Braque.

Another of last year's acquisitions, Theodore Gericault's classic lithograph "The Boxers" -- squaring off, black and white, symmetrically poised and opposed -- contrasts with George Bellows' lithograph of more than a hundred years later, "Dempsey and Firpo" in the modern ring, where form and light, asymmetrical figures and shiny sweat have become more important to the artist than exact musculature.

While works on paper are often relegated to second-class status, it's small wonder these are featured in the exhibition's first room, along with other '85 acquisitions: Degas' deft charcoal and pastel study, "Three Dancers Resting"; Mary Cassatt's dimpled darling in "The Black Hat," a pastel; and Maurice Prendergast's watercolor, "Revere Beach," with its flock of parasols perched on a dune.

Some of the British works here feel like holdovers from the "Treasure Houses of Britain" -- puffed-up, pre-Revolutionary dignitaries posing in a pastoral scene, a precious portrait of a family before its country house. America is well represented by, among others, a roomful of George Bellows paintings, and by some new (to the National Gallery) Winslow Homers.

Three of the French paintings have made their way back from the recent Impressionist exhibit. Some of the galleries used for that show seem scarcely to have changed: They remain plein air rooms visited by Madame Monet, or with views of two Waterloo bridges by Monet, or of young men plying limpid waters in their "Skiffs," by Caillebotte. This last one is new to the Gallery, along with Renoir's portrait of a pensive Monet reading and Matisse's "Pianist and Checker Players," a riot of textile design.

In all, about half the show comes from the recent acquisition. Such generosity might be difficult to understand. But, as Paul Mellon oversaw the building of the West Building and, with the help of his sister and a family foundation, paid for the East Building, it makes sense he would give the nation something to put in them. GIFTS TO THE NATION: SELECTED ACQUISITIONS FROM THE COLLECTIONS OF MR. & MRS. PAUL MELLON -- Opening Sunday in the National Gallery of Art's West Building, through October 19.