"American Portrait drawings," the beautiful exhibit that goes on view today at the National Portrait Gallery, was picked by Marvin Sadik, the gallery's director, a frustrated perfectionist at liberty at last.

Sadik has a first-rate eye; his gallery, however, has so limited a mission that one might never notice. His exhibits are obliged to contain portraits, often chosen only because of their sugjects. And the trouble with many of his shows is that excellent Americans have often been portrayed in pictures, which, if truth be told, aren't excellent at all. His gallery's displays often have been marked by superb publications, superb installations, and less-than superb art.

This one, for a change, is a beautiful exhibit on purpose.

With nearly 200 years of U.S. art to ransack, Sadik and his colleague, Harold Francis Pfister, have selected 96 American portrait drawings that share absolutely nothing save "esthetic excellence." One need not love portraits to see that this exhibit is a show of gems.

The faces that we see here could not range more widely. A few of them are famous -- here is Henry James, for instance, Washington Irving. Henry Adams -- but as many are unknown. The 1783 drawing by Benjamin West, with which the show commences, is a posthumous portrayal of the unfortunate Octavius, the 4-year-old, beloved son of America's last king, George III. oThe newest work on view is a too large and too-arrogant picture of himself which Alfred Leslie finished in 1977. In any show so large there are bound to be some losers: in this one there are very few.

The drawings here succeed because they somehow fuse two different sorts of beauty.The first sort is artistic. Almost all the pictures here, whether large or small, light or dark, sketched or highly finished, strike the eye, at once, as superbly drawn. But these objects are far more than well-marked sheets of paper. They also have about them a beauty that's psychological. When, in 1790, the masterful John Trumbull attempted to portray the Creek warrior, Hopothle-mica, the Indian resisted, so Trumbull, recorded under the impression "that there must be magic in an art which could render a smooth, flat surface so like to a real man."

To wander through this show is to experience the same awe. Some specially responsive portion of the mind makes of every man a connoisseur of faces. Beyond the varied markings on the sheets of paper here the viewer feels the presence of another human being.

Certain drawings shown -- by Eastman Johnson, John James Audubon (who is better known for birds), William M. Harnett (who is better known for still lifes), and Willem de Kooning -- have a presence so compelling that they seem to glow upon the wall. The Sheeler and the Gorky, the Leutze and the Chalfant, also are impressive. This show contains surprises. We knew that Thomas Eakins was a major master, and his little portrait here of Dr. Gross of the Gross Clinic, done with little scribbles, washes and cross-hatchings, makes that point again. But no one who has frowned at the slightly hokey paintings of Joseph Stella (1877-1946) will fail to be moved by the three quite different portraits that represent him here.

Though a number of these draftsmen -- Franz Kline and de Kooning, Edward Hopper, yandrew Wyeth -- have mighty reputations, there are other portraits here, comparably impressive, by Theo Wujcik, Leo Dee, William Bailey, Don Demauro artists of great talents who are not so well known.

The weakest works displayed -- the rough Elaine de Kooning, the flabby Jamie Wyeth -- are not weak enough to do much damage to this show.

As so often is the case with Sadik's publications, and those of Pfister, too, the accompanying catalog with its full-page illustrations is a first-rate piece of work. In most of its efforts, the Portrait Gallery functions as a museum of people, not of art. But when it mounts, as it has here, exhibits that are meant to serve the beautiful it does as well as any art museum in the land. Would that Sadik would arrange a show as fine as this one at least once a year. "American Portrait Drawings" closes on Aug. 3.