The Hirshhorn Museum and Sculpture Garden began adding to its holdings, and its list of donors, when it was still new. Through gift, through purchase, and, in one case, through exchange, the museum has acquire 59 paintings, 18 sculptures and 58 works on paper since it opened to the public in October 1974.

All these objects are included in "Acquisitions: 1974-1977," which opens there today.

The show should please Joseph H. Hirshhorn. Though tiny in comparison, it is as inclusive and uneven as the vast collection that he gave the nation 11 years ago.

Hirshhorn's tastes were catholic. Unlike Andrew Mellon, say, he was a democrat of a collector. He did not seek only masterworks. He also bought his share of minor things and footnotes to the history of art. The museum has collected in the same spirit. It has deepened and extendel, but it has not contradicted, Joseph Hirshhorn's gift.

The Hirshhorn is not rich. It has been given only $248,000 (or slightly less than $100,000 a year) for new acquisitions. The money, provided by the Smithsonian Institution, has been spent with care.

The museum has purchased 34 works of art - average price: $7,300 - and some of them are splendid.

Among the most impressive are major metal sculptures by Anthony Caro and Claes Oldenburg; canvases by Robert Cottingham, Jules Olitski, Alice neel and Stuart Davis; and smaller works on paper by Frank Stella, John Storts, Oscar Bluemnes, Louis Lozowick and Paul Sarkisian.

Certain new directions are apparent in the show.

When Hirshhorn gave his gift in 1966, photo realism was not as fashionable as it is today. The Cottingham and the Sarkisian are thus significant additions to his vast collection.

The Hirshhorn is both a national and a Washington museum. A number of local painters are represented in this show. They include Gene Davis, Howard Mehring, Edward Corbett, Anne Truitt, Willem de Looper, Alma Thomas and Jacob Kainen.

The exhibition also relects temporary exhibitions staged by the museum. Works by E. E. Cummings, Peter Plagens, Hsiang-Ning Han, Jack Bush, John Covens, Lozowick and Hans Richter have been acquired from its shows.

The Hirshhorn differs from most museums of modern art in that it places equal stress on pictures and on sculptures. A number of paintings, prints and drawings by important sculptors - David Smith, John Storrs, Elie Nadelman, Henry Moore - are included in the show.

Among the smaller works displayed are a number of delightful footnotes - a small oil by Allan Kaprow, who made his name through happenings, a drawing of Henry Geldzahler by England's David Hockney, a Grandma Moses farmscape, and a small portrait of collector John Quinn by John Butler Yeats, the poet's brother.

Because their quality is so high, certain works - by Olitski, Joan Mitchell, Caro Bluemner, Stuart Davis, Richter - dominate one's memory of this varied show. But it is not free of failures, Pictures here by Gandy Brodie, Pat Adams and Leon Harti seem, to me at least, less beautiful than ugly, The weakest works are gifts.

The museum, when it opened, had a single donor. It now has 35. Hirshhorn is still giving, but he has been joined by Dillon Ripley, Max Protetch, Sen. and Mrs. Dale Bumpers (D-Ark.), and otehrs. Though it often turns them down, the Hirshhorn welcomes gifts.

Though important gaps - conceptual art, for instance - remain in its collections, the Hirshhorn is alive. "When we opened," notes Abram Lener, its director, "Washington did not have a true museum of modern art. We have to continue growing. We can't let that happen in the future."

To be a mustum of modern art, the Hirshhorn must expand. This exhibition demonstrates that its director, curators, donors and trustees have made an admirable start. "Acquisitions: 1974-1977" closes July 24.