It was called the "Temple of Invention" when it opened as the U.S. Patent Office more than a century and a half ago, and it became a temple of art in the 1960s. It is one of Washington's architectural treasures, and it will close on a staggered schedule next week for a major, three-year renovation.

This weekend, then, is pretty much the last opportunity to say temporary goodbyes to the Old Patent Office and to the two Smithsonian museums that have shared its handsome spaces since 1967--the National Museum of American Art and the National Portrait Gallery.

Each museum, as it happens, is finishing with a flourish. "Edward Hopper: The Watercolors" has been luring sizable crowds to the American Art Museum, which officially shuts its doors at Monday's end. "Tete-a-Tete: Portraits by Henri Cartier-Bresson" is the star attraction at the Portrait Gallery, which closes Sunday, Jan 9.

Visiting the museums before they close is a good idea not only because the art and the architecture are rewarding, but also because both the museums and the building will be quite different when they reopen in 2003. In some ways they will be measurably improved, and in other ways perhaps not.

Many of the most important improvements--the replacement of 35-year-old electrical and ventilation systems--will not be all that visible. It will be delicate work, boring through thick masonry walls for new conduits and finding spaces for heavyweight boilers and chillers. But the intention of the renovation architects--the redoubtable Washington firm of Hartman-Cox--is to make it look as if almost nothing had been done to the historic structure. That is, of course, as it should be.

But other changes will be obvious. Access for disabled visitors will be greatly improved with the installation of new elevators. Nasty dead ends will be eliminated, so that, for the first time since the '60s conversion, visitors will be able to circumnavigate the rectangular building on its grand second and third floors.

Most important, there will be more--lots more--exhibition space open to the public. The Portrait Gallery will gain nearly 6,000 square feet, the American Art Museum nearly 28,000, for a total gain of exactly 33,817 square feet.

Clearly we are not talking small change here. Imagine, for instance, a trip to the Corcoran Gallery of Art in which you walk through every single gallery (excluding, of course, the many that have been converted to storage and other uses) Okay, you have done it, and you are suffering from pleasant museum fatigue. You have just completed a tour through 29,000 square feet of exhibition space--less than the space gained by the American Art and Portrait Gallery renovation.

The combined package--a total of 128,116 square feet for galleries--is an increase of more than a third. This impressive gain was made possible by the Smithsonian's purchase of the nearby Victor Building and an adjacent downtown property at Ninth and H streets NW, where a new building is under construction. Both the Portrait Gallery and the American Art Museum will move most of their offices and support facilities across Ninth Street, freeing up space in the Old Patent Office building.

With so much at stake it is not altogether surprising--but wholly regrettable--that a bitter turf war broke out between the two museums during preparations for the renovation. I. Michael Heyman, then secretary of the Smithsonian, wisely nixed the idea (forwarded by Museum of American Art Director Elizabeth Broun) of merging the two, for they are quite distinct--the Portrait Gallery is primarily a history museum and the American Art Museum, as its name announces, is mainly about art.

On the other hand, Heyman's decision on how to divvy up the space is likely to cause confusion. Of course, having two museums share one building has always been a bit perplexing, but the current, Janus-faced arrangement, with American Art mainly on the north and the Portrait Gallery altogether on the south, makes the distinction reasonably clear.

Under the new arrangement, by contrast, the north-south "barrier" has been broken definitively.

If you enter from the north off G Street, you'll be in the Portrait Gallery on the first floor and American Art directly overhead. If you enter from the south off F Street, you head east for portraits and west for a section of American Art and then, on the second floor, you head east for American Art and west for portraits, and then, on the third floor . . . well, never mind.

Let's just say that the results will be far from perfect while admitting that perfection, in this divided context, is impossible. In any case, there also are other things to worry about, most of which can be summed up in a single word. Money.

The Smithsonian decided to allot $60 million to the project. This is enough, says a Smithsonian spokesman, to pay for a renovation of the basic building systems and make the building "a first-class facility." On the other hand, it does not cover such things as restoring marble floors in spaces like the sweeping Lincoln Gallery, solving an age-old problem on the north side by building an attractive staircase (or an escalator) from the first to the second floor, or replacing the grand staircase to the south portico, shorn off some 60 years ago.

It is symptomatic of our times, I suppose, that such things are considered to be aesthetic frills--and a great shame, for this renovation is as close as we will come to a Heaven-sent opportunity.

The Smithsonian bureaucracy isn't against such improvements, mind you, but expects them to be paid for by "nongovernmental" sources. This puts a lot of pressure on the directors and boards of the two museums, and makes the infighting all the more reprehensible, for what should be happening apparently is not happening: That is, the two institutions are not collaborating to raise private money to pay for such desirable improvements.

About the two proposed staircases, there is this to say: In the north: No escalators, please. Or, if there are to be escalators, please tuck them out of sight, for they are wildly out of place in this context. This is one of America's greatest 19th-century buildings.

In the south: Please give the idea of adding a grand exterior stairway serious consideration. The story is that the original stairway was replaced after F Street was lowered substantially during "Boss" Alexander Shepherd's citywide campaign to improve Washington's streets in the 1870s. The replacement, not quite as proportionate as the original, was cut away in the 1930s when the street again was lowered.

Ever since, that fine Greek Revival portico, modeled after the Parthenon, has looked a bit like the end of a giant sliced sausage. The Smithsonian wisely insisted upon a sidewalk wide enough to accommodate a new set of stairs during a recent renovation of F Street. The next step would be to ask Hartman-Cox actually to design such a thing, so we can judge the new proportions.

My feeling is, the proportions will turn out fine. The building was designed to be entered on its grand, high second story, and this is how it should be when we return again three years from now. And then we can turn left for portraits and right for American art. Or is it the other way around?

CAPTION: A bust of Andrew Jackson in the National Portrait Gallery, which along with the Museum of American Art is closing this month for renovations to the Old Patent Office.

CAPTION: The F Street side of the Old Patent Office: Ever since its stairway was removed, the Greek Revival portico has looked a bit like the end of a giant sliced sausage.

CAPTION: A stairwell in the Museum of American Art is home to Frederick Judd Waugh's "Knight of the Holy Grail."