About 600 works of art - among them the first drawing by Italian master Andrea Mantegna (1431-1506) to enter an American collection - were acquired by the National Gallery of Art between July 1, 1975, and Sept. 30, 1976.

The acquisitions include some treasures - by Ingres, Eakins, Tobey, Francis Bacon, Kenneth Noland, Tony Smith and Tintoretto - but most will not be shown until the Gallery's East Building opens here next June.

When the current exhibition of the cut-outs of Matisse closes on Oct. 23, the Gallery will enter what its director, Carter Brown, calls "a period of apparent quiet." No other major loan shows will open there this year.

Behind the scenes, however, the museum has been steadily improving its collections, and its recent acquisitions are listed in the Gallery's just-published 1976 annual report.

The four-inch-high Mantegna, which shows a small bird perched upon a branch, pecking at some berries, seems a modest picture - but there are only nine sure drawings by Mantegna. All the others are in Europe, and this one, from the Gathorne-Hardy collection, was the last in private hands.

It was puchased by Colnaghi's for 60,500 pounds (about $105.000) at Sotheby's in London at April 28, 1976. The previous auction record for an Old Master drawing was the 58,000 pounds fetched by an Albrecht Durer in 1969. The Gallery will not disclose how much it paid Colnaghi's, but those dealers, it seems likely, took their usual commission.

Other significant acquisitions listed in the Gallery's annual report include:

John Singleton Copley's portrait of Harrison Gray, a gift of the Hon. and Mrs. Robert H. Thayer.

Thomas Eakins' portrait of Harriet Husson, Carville, a gift of Elizabeth O. Carville.

Francis Baco's 1954 "Study for a Running Dog," given by Ruth Fisher Rhetts.

"The Clown" by Kenneth Noland, a 1959 "target" (with strips), a gift of Dr. and Mrs. Robert J. Wetmore.

"The Sanctuary of Hercules" by Arnold Boecklin (1827-1901). This moody evodation of the Glory that was Greece is the first important German Romantic painting to enter the Gallery's collection. It was purchased, as was the small Mantegna, with monies from the Andrew W. Mellon Fund.

"New York," an early (1944) allover field painting by American Mark Tobey, paid for by the museum's Avalon Fund.

"Red Rose Cantata," a 1973 Washington color painting by Alma W. Thomas, a gift of Vincent Melzac's.

A large untitled canvas painted in 1962 by sculptor Tony Smith, a gift of Mr. and Mrs. Burton Tremaine.

The Gallery also acquired nine sculptures, among them a 1965 glass box by Larry Bell (another gift of the Tremaines), a 1774 allegorical terra cotta, "Poetry and Music," by Claude Michel Clodion (bought through the Loula D. Lasker Fund). "Two Nudes," a 1911 plaster relief by Elie Nadelman (a gift of Robert P. and Arlene R. Kogod), and a large granite Noguchi, "The Great Rock of Inner Seeking," which entered the collection as an anonymous gift.

The report also notes that Leni Stern of Washington has promised to give the Gallery "Cubi XXVI," a stainless steel sculpture by the late David Smith.

In addition, the Gallery acquired more than 500 prints and drawings, of which the report describes the small Mantegna as "by far the most important drawing purchased." "Outstanding among the gifts of graphic art," it continues, "were Mrs. Charles R. Henschel's legacy of 17 brilliant Winslow Homer watercolors . . . Mr. and Mrs. Robert H. Smith gave a beautiful Ingres portrait drawing of Auguste-Jean-Marie-Guenepin, Ingres' friend and fellow student at the French Academy in Rome in 1809. The Woodward Foundation made an extremely important addition to the Gallery's modern collection . . .For example the gift included 47 prints by Jasper Johns and 31 by Robert Rauschenberg . . . Among other gifts received were . . . 21 1970 lithographs by William de Kooning from Lee and John Eastman.

"Other fine drawings included our first Tintoretto; a Rubens copy of a Parmigiano etching after a Raphael cartoon - an extraordinary lineage; an atmospheric Claude landscape of the 1950s . . .; Marcoussis' cubist portrait of Guillaume Apollinaire; and Joseph Cornell's collage based on the National Gallery's Dosso Dossi painting "Circe and Her Lovers."

A small number of these prints and drawings have been placed on exhibition in the Gallery's print study rooms, where visitors can see them, but only by appointment.

Though the Gallery issues press releases for occasional major acquisitions - Jackson Pollock's "Lavender Mist," for instance, was purchased in 1976 - other additions to its collections are listed for the first time in its annual report.

This one, for example, notes that in the period covered the museum's art library had grown to 41.895 titles in 52.743 volumes: that "the total number of photographs received between July 1, 1975, and Sept. 30, 1976 was 155.984," and as of the latter date, the Gallery had a roster of 676 employees.