The last time Ronald Reagan was inaugurated, Armand Hammer observed the occasion by displaying his newly purchased Leonardo notebook. He was given exhibition space by the Corcoran Gallery of Art. This time he will show 16 American pictures from his vast collection. They will go on view on Sunday in the National Gallery's West Building.

"American Paintings from the Armand Hammer Collection: An Inaugural Celebration" is a quickly put-together little grab bag of a show. It will mystify the viewer. Its scattered choices are perplexing. It mixes splendors and embarrassments. It suggests the man himself.

Hammer, 86, is at ease with contradictions. He is a capitalist -- head of Occidental Petroleum -- a billionaire who gets along just fine with the rulers in the Kremlin. (Recently he met there with Konstantin Chernenko. They discussed Post-Impressionist paintings and nuclear first strikes.) He regularly spends fortunes on costly works of art he rarely gets to look at. No sooner are they his than he sends them out on loan. He has given away large sums of cash. He is a generous philanthropist -- and a ceaseless self-promoter.

One reason the Corcoran and the National have agreed to host his inaugural "celebrations" is that both museums are in Hammer's debt.

The Corcoran's auditorium, whose refurbishment he paid for, bears his name. He has also given that museum many Daumier prints. Of his many gifts to the Corcoran, perhaps the most important was the grant that enabled it to dispense with admission fees. Meanwhile, Hammer has pledged his works of art on paper -- worth many millions of dollars -- to the National Gallery of Art. Two of them -- a fine Mary Cassatt pastel and an Andrew Wyeth landscape -- are among the 16 pictures in the gallery's show.

"In their quality and diversity," observes J. Carter Brown, the gallery's director, "these paintings reflect Dr. Hammer's lifelong dedication to collecting and exhibiting great art." Brown is also being generous; one trips on that word "great."

In the late 1960s, Hammer began to improve significantly the quality of his collections. Before that he often bought second-rate paintings. He still does so sometimes.

Two are in this show. In 1980 he purchased a surprisingly crude Charles Marion Russell of Indians on the warpath that had been sold off, for good reason, by the Cowboy Hall of Fame. Nearby hangs a Gilbert Stuart portrait of an Irish child that Hammer bought in 1982. It is even worse.

The Stuart apes the hoity-toity style of Gainsborough or Romney. It shows a little boy, George Thomas John, later eighth earl of Westmeath, posing with his collie. The collie, whose snout is portrayed as having the shape and softness of a beer bottle, is chewing on what appears to be a bloody handkerchief. Has it been savaging the sheep?

Of course Stuart is one of America's most important portraitists, and two good examples of his art -- one a likeness of the little boy's father, the other an even better portrait of George Washington -- are in Hammer's exhibition. It is the contrast between these Stuarts that will amaze the viewer. The Washington is grand. The child's portrait is a dog.

The show also includes two superb full-length portraits by John Singer Sargent, a fine life-size Thomas Eakins (of Sebastiano Cardinal Martinelli) and two still lifes by William Michael Harnett (one of which Hammer bought from J. William Middendorf II and donated to the White House in 1978). Also on display are a Whistleresque Frederic Remington of a singing cowboy and an oddly awkward, voyeuresque Andrew Wyeth nude. These artists are so famous that one is surprised to find among their pictures a scene of the French countryside by a far less known 19th-century academic, Daniel Ridgeway Knight.

Hammer is not one of those collectors interested primarily in the joys of connoisseurship. In this show, and others, one gathers he buys works of art for various other motives: they link his name with masters; they display his generosity; they keep him in the news.

Many of its paintings have been seen here before. Half of them -- including Stuart's Washington, the Sargents and the best of the Harnetts -- were shown at the Corcoran in 1980. The exhibit, which closes Feb. 18, would feel like an embarrassing payoff to a donor were it not subtitled "An Inaugural Celebration." Indeed, frivolity is permissible at parties -- there's nothing wrong with hanging streamers in the gym.