Today, for the first time since the National Gallery of Art opened in 1941, the awesome bronze doors at Seventh Street will slide open for keeps. Behind the doors is what gallery director J. Carter Brown refers to as a "museum within a museum."

For many visitors, Brown's characterization will seem understated. A whole new world of art objects, nearly 2,000 in all, has been installed in a sequence of galleries beautifully reclaimed from what used to be a warren of storerooms, offices and galleries tucked away in the West Building's ground floor.

These spaces were shut down one by one more than a decade ago as construction began on the gallery's East Building, which opened in 1978. Since then work has proceeded gradually on the $16.7 million "Operation Breakthrough," a complete reconditioning of the original building's lower floor to allow greater public access and display of many of the museum's too-long-hidden treasures.

Viewers who frequented the gallery in the years before the East Building expansion began will again be able to see rare, favorite art works--the medieval chalice of the Abbot Suger of Saint Denis, the peach-blossom Chinese porcelains from the Widener collection, the famed Mazarin tapestry of the "Triumph of Christ"--but the overall effect is of a tremendous new gift to the city of Washington and its many visitors.

The key elements of the new West Building package are sculpture and the graphic arts, which for the first time are given cohesive, chronological presentations that echo the great march of Western paintings on the West Building's main floor.

To be sure, there are plentiful, amazing diversions. These include a spacious display of 63 American naive paintings from the more than 300 such works donated by the late Edgar William and Bernice Chrysler Garbisch; the 18th-century-style French rooms, with their selection of exquisite furniture from the best cabinetmakers in royal employ, which came as part of the Widener collection; and the 176 Chinese porcelains, again installed in their elegant, built-in display cases. (In former times, when this room was a popular shortcut, curators called it "going through China." Now, as a staff member quipped, "China has been rediscovered.")

Nonetheless, it is in the new graphics and sculpture galleries that one begins to feel the strong pull that Brown describes as a "subtle dialogue between the downstairs and upstairs" of the museum.

"It is something like a great record collection," he says, "with a section for symphonies and a section for quartets. They are in separate places, but many of the best things are by the same composer. In the upstairs galleries people will see paintings by Boucher, and downstairs they will see a tapestry based upon his work. They will see Renoir as a painter and also as a sculptor, and Rembrandt as perhaps the greatest etcher of all time."

"Operation Breakthrough" was accomplished in phases. To accommodate the plaza between the West and East buildings the Fourth Street lobby was remodeled in 1976 with a balcony and a sleek clear-glass entranceway. Next came the long spine of arches (designed by David Condon of the Washington firm Keyes, Condon and Florance) that today defines the sales area on the ground floor of the old building. In the process a new restaurant, the Garden Cafe, was added and the conservation, receiving and photographing departments were relocated and rebuilt.

Today the job is complete. The crisp east-west axis running from the dramatic central fin of I.M. Pei's East Building directly through the middle of John Russell Pope's grand neo-classical structure is now open all the way to Seventh Street on the ground floor as well as the main concourse of the older building. As a result 40,000 square feet of new exhibition space have been opened on the ground floor.

Upon entering from Seventh Street the first thing one sees, looking east through two rounded archways (artfully made from plaster to give the feel of stone), is a standing white marble sculpture, "Calliope," by an 18th-century French sculptor named Augustin Pajou. From her dramatically lit perch she is a beacon luring viewers down a slight incline and into a sequence of spaces, nearly 40 galleries in all, that is altogether more intimate than the parade of high, paneled rooms upstairs. This fits the art on view.

When Andrew Mellon first conceived of a national museum in Washington, he thought of it mainly as a gallery of great pictures. Late in his life, however, influenced mightily by the all-persuasive dealer Lord Duveen, he began to add distinguished examples of Renaissance sculpture to the core collection he would donate to the nation. As a result the National Gallery became one of the few institutions in the world to fully integrate sculpture into its permanent displays.

Many of the larger, more eminent Italian sculptures remain on view in the upstairs rooms of the old building. Downstairs, starting today, there is room after room of smaller works, many of comparable distinction.

The gallery's collection of Renaissance medals, plaquettes and relief sculptures, for instance, is unquestionably one of the greatest in the world. Only slightly more than half of the some 1,500 objects in this treasure trove have been put on public display--the rest await a connoisseur's closer inspection in new study rooms--but these include some of the rare gems of the genre: Leone Battista Alberti's self-portrait in bronze relief (c. 1435); great relief bronzes such as Riccio's "The Entombment" (c. 1505-1515) or "Orpheus and Euridice" (1515) by Peter Vischer the Younger; and numerous medals struck by Pisanello, the first Renaissance medalist.

Unlike the medals of classical antiquity, Pisanello's medals and those that followed in profusion in the century or so after he died in 1455 were made for individual princes, princesses, soldiers and merchants. Although too numerous to take in during a single inspection, they are, as curator Douglas Lewis advises, "one of the best ways to understand the humanism of the Renaissance because they had to do entirely with human beings."

The sculpture galleries continue through the centuries in rather scattershot fashion but with extraordinary pieces, such as four marble busts by Houdon; a stupendous tiger by Antoine Louis Barye, the 19th-century French animalier; an amazing set of wax figure studies by Degas that look as if they had just left the great artist's hands; a superb selection of Rodin marbles and bronzes; and, as they say at the circus, much, much more.

For sheer quality piece by piece, however, the display of prints would be hard to match. It seems clear in the first few moments in the first of the chronological print rooms that curator Andrew Robison decided simply to knock our eyes out with the finest impressions by the finest artists in the gallery's substantial collection of graphics in the Western tradition.

The display begins at the very beginning with a crisp woodcut of a saint by an anonymous German printmaker (c. 1450) and continues on the same wall with absolutely first-rate examples by the best in the northern world: "The Death of the Virgin" (c. 1470) by Martin Schongauer, a "Madonna and Child Enthroned" (c. 1465) by the Master E.S., and three prints by Albrecht Du rer that prove his epitaph to be entirely deserved. "All that was mortal about Albrecht Du rer is covered by this tomb."

On the opposite wall is Italy in prints of comparable fame and worth: Antonio Pollaiuolo's "Battle of the Nudes" (c. 1470), Andrea Mantegna's "Battle of the Sea Gods" (c. 1485), Giulio Campagnola's "St. John the Baptist" (c. 1505), and Parmigianino's "The Entombment" (c. 1530). This is indeed a high level and a brisk pace, and it is only the beginning. At least until we reach the 20th-century room the pace and level are maintained.

This chronological display of the graphic arts is to be a permanent part of the National Gallery's presentation, although, for reasons of conservation, the examples will be rotated every six months. It is by no means the end of the graphics exhibitions, however. Also on view is a breathtaking room of recently acquired drawings and, in galleries allotted to temporary exhibitions, displays of 170 photographs by Alfred Stieglitz and a loan show of rarities, "Drawings from the Holy Roman Empire, 1540-1680."

Clearly it will take years for even a dedicated viewer to assimilate all of the material in the new "museum within a museum," which all at once adds several new dimensions, and reintroduces several old ones, to the National Gallery of Art.

The $16.7 million project was paid for by The Andrew W. Mellon Educational and Charitable Trust ($5 million); Paul Mellon ($4.5 million); the federal government ($3.7 million); the late Ailsa Mellon Bruce ($1 million); The Andrew W. Mellon Foundation ($1 million); the Kresge Foundation ($1 million); and receipts from sales in the National Gallery stores ($500,000).