The National Gallery of Art is growing once again.

Out of public view in the bowels of its West Building, construction has begun on what director J. Carter Brown calls "Operation Breakthrough." By 1982, the project will add two new museums to the Gallery's ground floor at Seventh Street NW, accommodating the arrival of nearly 30,000 prints which will double the number of those now on hand.

One of these new "museums-within-the-museum" will exhibit bronzes, medals and small sculptures rarely seen before. The other will be "a first-rate new museum of the graphic arts," Brown says, to be used for drawing shows, print shows and a regularly changing historical survey of important works on paper.

Two recent developments triggered the expansion.

First, the completion of the East Building and its adjacent Study Center has emptied perhaps half of the West Building's ground floor. New space for staff offices, the library, the print rooms and the lab, as well as huge new galleries for temporary shows, have now been provided in I. M. Pei's East Building. Some who have complained that Pei's design appears to offer relatively little room for exhibitions may not have considered the space in the West Building that his construction has set free.

The other development was the death in June -- at the age of 88 -- of collector Lessing J. Rosenwald of Jenkintown, Pa.

"His was the greatest single collection of prints, drawings and illustrated books ever formed by one man in the United States," says Andrew Robison, the Gallery's curator of prints and drawings. Rosenwald began giving his collection to the nation more than 30 years ago, but while he lived he kept it in the small museum that he built adjacent to his home.

According to Rosenwald's wishes, his extraordinary holdings will now be moved to Washington. His rare books have been given to the Library of Congress. His choice drawings and his prints -- almost 30,000 -- soon will be transferred to the National Gallery of Art.

One can only guess at the value of Rosenwald's gift, for he began to buy old prints in the 1920s, and no comparable collection could be formed today. Perhaps a dozen early 15th-century woodcuts come up for sale in a year; Rosenwald owned hundreds. Martin Schongauer's rare engravings, when they come up at auction, today might fetch as much as $200,000; Rosenwald bought 81. His fine and broad collection, with its Durers and Picassos, is a superb survey of the history of printmaking, and will approximately double the number of prints on hand here for study and display.

Rosenwald, once board chairman of Sears, Roebuck & Co. (his father had purchased a quarter of the firm for $37,500 in 1891) began giving to the Gallery in 1943.

It is from these two dovetailed collections, the arriving Rosenwald prints and the one the Gallery formed to fit his previous gifts, that the displays in Brown's "museum of the graphic arts" will eventually be drawn.

When "Operation Breakthrough" is completed, an open "spine" of galleries, shops and orientation spaces will run from Seventh Street to Fourth Street on the Gallery's ground floor. The visitor who enters at the Seventh Street Lobby (now closed to the public) will be able to see the new East Building three blocks away. Proceeding down the spine, the new museum of small sculptures will be on the left, the museum of the graphic arts on the right. One of the displays there, the survey of printmaking, will parallel the chronological painting survey on the floor above.

"If you come to the Gallery you see less than 10 percent of Rembrandt's paintings," says curator Robison. "When the Rosenwald gift arrives, we will have on hand here impressions of perhaps 90 percent of Rembrandt's prints."

The Gallery's main Sixth Street entrance now appears to lead to nothing, except to the post card shop and the restrooms beyond. When the first phase of the project is completed, a new "orientation center" will be opened just inside the door, and the viewer who turns left will have a view of daylight through the glazed Fourth Street lobby nearly two blocks to the east.

Little natural light will be provided in the graphics galleries themselves, but that is just as well. Works on paper fade in light, and individual examples will be shown for short times only, but the collection, thanks to Rosenwald, is now so extensive that a constantly changing survey will be always on display. No such chronological survey of the history of printmaking from the 15th century to the present is now on display in an American museum.

"Most great museums have their paintings on display," says Brown, "but if you want to see their prints you have to make appointments, and study them in privacy while some suspicious crone stands by, breathing down your neck. We want to have a survey up that everyone can see."