The National Gallery of Art will celebrate its 50th birthday in March, and has invited 150 of its closest friends to come and bring presents -- preferably their greatest works of art. Friends without art have been invited to bring money -- $50,000 minimum, to help buy other coveted objects. Brazen or bold, this strategy has already bagged a breathtaking 550 new works and $12.5 million in cash.

And what presents! Pamela Harriman, as promised, is bringing her white roses by van Gogh, and Betsey Cushing Whitney her late husband's glorious Toulouse-Lautrec painting featuring the red-haired actress Marcelle Lender dancing the bolero in the comic operetta "Chil Peric" -- arguably the greatest painting by the artist still in private hands. Paul Mellon, son of the gallery's founder, will bestow his usual cornucopia of wonders, among them Cezanne's famous "Boy in a Red Waistcoat," with a Cezanne sketchbook of 73 drawings tossed in as lagniappe.

Such postimpressionist treasures may well steal the show when the exhibition of birthday loot -- "Art for the Nation: Gifts in Honor of the 50th Anniversary of the National Gallery of Art" -- opens March 17. But there are many gifts still to be announced, including two Monets, a Chagall and works by Avery, Rauschenberg and Johns. Whatever comes, center stage will have to be shared with at least one incomparable American masterpiece: Winslow Homer's iconic 1877 watercolor of a young woman standing before a blackboard, the jewel of the Los Angeles collection of Jo Ann and Julian Ganz Jr.

Together with other works dating from Jacopo Bellini's 1459 Venetian panel painting of two saints to Wayne Thiebaud's 1963 painting "Cakes," from a portrait drawing by Jacques-Louis David (from Walter H. and Leonore Annenberg) to a portrait painting by Thomas Eakins (from Sen. and Mrs. H. John Heinz III), the upcoming show of 320 of these gifts may well represent the greatest birthday hul of the century.

How did they do it? "Piece of cake," says National Gallery Director J. Carter Brown. "It's an extraordinary outpouring of generosity." By way of example, Brown has added his own gift to the celebratory heap: the gallery's first drawing by Titian.

Brown's gift was announced yesterday, along with 17 other old master paintings, watercolors and drawings, prints, one sculpture and one extraordinary book, the 1493 Nuremberg Chronicle, another Mellon gift. It was the first of three batches of birthday gifts and newly purchased works to be unveiled before the exhibition opens in March.

None of the old masters could -- or does -- rival the Raphaels and Botticellis given by Andrew Mellon when the gallery first opened in 1941. But all fill gaps in the National's permanent collections, especially in the realm of Dutch and Flemish still life and Italian baroque, according to Senior Curator Andrew Robison, who headed the staff committee set up to seek out and coordinate solicitations for works of art.

A still life with flowers by Jan Brueghel the Elder, promised by Bunny Mellon, goes a long way in the still life category. And shoring up the gallery's current penchant for the baroque is the "Martyrdom of Saint Bartholomew" by Jusepe De Ribera, recently purchased with $5 million of the $5.8 million raised specifically for that purpose by the Fiftieth Anniversary Gift Committee.

That committee -- which focused on raising cash while curators, trustees and Brown himself wooed collectors for art -- was headed up by Washington real estate mogul Robert H. Smith, a National Gallery trustee and president of the Charles E. Smith Building Corp.

Brown credits imaginative and hard-working trustees such as Smith with the success of the birthday campaign. "This is all new for the National Gallery," he says. "In the old days, we never had breadth of support like this. Because of the prestige of this place, we got a response I frankly was surprised by.

"In the process, we found that Bob Smith was a wonderful fund-raiser who set his sites on the local community, where we hadn't really looked before," says Brown.

In fact, Smith's success is likely to come as a shock to many who have tried to raise funds for other cultural institutions in Washington, a city notorious for being tight. He brings the startling news that 55 percent of the $5.8 million raised by the anniversary committee came from area residents and corporations, along with a respectable proportion of the art, roughly half of which consists of works on paper.

But Washington was by no means the focus here. This birthday celebration is the culmination of a 10-year effort to broaden the museum's constituency to a national base -- to convince great collectors across the country that the National Gallery, as well as their own area museums, deserve their support.

Do museums elsewhere in the country mind that some of their key collectors responded to the National Gallery's request, possibly to the loss of hometown museums? The director of one major California museum asked not to be named, but was obviously miffed. "Let me be polite," he said. "I'd rather not comment."

"We've been very explicit," says Brown. "This campaign has been to represent collectors by a single object. I've been frank with people, that they should try to support their own museums. But once that's established, it's hard to say that there isn't a single object they can spare for their country."

Dealers, however, have been murmuring happily for the past year about gallery curators flush with cash, grabbing up treasures. To an extent it's true: Several donors, rather than giving cash, chose to buy works on curatorial "wish lists." Such gifts -- along with others earmarked for special anniversary films, publications, concerts and other events -- make up the rest of the $12.5 million.

Widening the Circle For the past decade, the gallery has been widening its circle of friends. "The National Gallery has only five trustees," says Smith, "which means that for a long time, it's had a very small constituency. Most museums have 30 or 40 or 50 people, and you're able to reach out." In addition to Smith, the trustees are Chairman Franklin D. Murphy, director emeritus of the Times Mirror Co. of Los Angeles; President John R. Stevenson, counsel, Sullivan and Cromwell, Washington; his wife, Ruth Carter Stevenson, president, Amon Carter Museum and Foundation; and Alexander M. Laughlin, portfolio manager, Deltec Securities Corp., New York.

To expand its reach, in 1982 the Trustees Council was formed, a group of about 35 people who have an interest in and support the National Gallery. Also, 10 years ago, Ruth Carter Stevenson formed the Collector's Committee, whose 100 members now each give $10,000 a year for contemporary art acquisitions.

"We felt we needed even a broader base," says Smith, who with Katharine Graham, chairman of The Washington Post Co., then launched the Circle of the National Gallery, which now has 320 members who each pay from $1,000 to $4,000 annually "for things that aren't normally in our budget. ... We now bring in excess of $500,000 a year where just a few years ago we had nothing."

In his fund-raising, Smith's chief weapon has been lunch in the director's dining room at the National Gallery -- lunches for 12 that already have a certain legendary status. "When Bob invites you to lunch," says one friend, "you know it's going to cost you at least $50,000."

But Smith is a powerful advocate. "When you stop to think of the National Gallery, and what hangs within its walls, and that it's only 50 years old, that is extraordinary," says Smith. "If you go to the Kunsthistorisches Museum in Vienna, or the Louvre in Paris, or the Rijksmuseum in Amsterdam, or the National Gallery in London, you're talking about collections and entities that were formed 150, 200 years ago. Yet here, in 50 years, the greatness of the National Gallery has been secured."

"He can be very convincing," says Carter Brown.

Though other trustees held dinners all over the country, this is where the fund-raising started, says Smith. "It began with people in business who recognize that they've had financial success in Washington, that Washington is the capital of the United States, and that the National Gallery represents the epitome of quality, an institution whose standards are so high that people want to be associated with it."

"You often give a check to an entity, and you never know what's going on. Here, you can go to that museum seven days a week, and it's giving something back on a regular basis -- not just through the paintings hanging on the wall, but through its education department and lending service, which loans paintings to smaller museums throughout the country, through tours, through cassettes, through the catalogues it publishes. The National Gallery is a great educator, and is working full time to make a real contribution to the American people. And that kind of a cause is the easiest kind to sell... .

"The minimum we were soliciting was $50,000," says Smith, "and we really were trying to get as many $100,000 gifts as possible." The majority, he says, were in the $100,000 range. "At that rate, we didn't need thousands of contributions. If you say that the average gift was $75,000, you need 70 gifts."

What did they get for their money? "Very few people made the gift for the lunch," says Smith. "They did it for the identification with the 50th anniversary." Their names will be inscribed on a plaque in the founders room. It now costs $1 million minimum to get your name carved in marble.

Smith has not only been "carved," he's also backed up his talk with 18 gifts of art over the last decade, most of them old master drawings. With his wife, Clarice, an artist, he has also given two birthday gifts of art: a 16th century Flemish painting by Roelandt Savery and a drawing by the 18th century Venetian master Canaletto .

The Tax-Law Window Since its founding, the National Gallery has relied solely on the generosity of individuals for acquisitions of the masterpieces that hang on its walls. No federal funds can be used to buy art. But because tax laws since 1986 have made gifts prohibitively expensive for very wealthy donors, the "partial gift" has evolved, in which a percentage of a work of art -- say, 10 percent -- can be given, and deducted, each year.

Until recently, it looked as though nearly all gifts in this anniversary show would be partial gifts. But Congress voted a one-year change in the tax law, which in 1991 permits donors to deduct the fully appreciated value of works of art without penalty. Possibly thanks to that one-year window, 200 of the 320 works representing the cream, and bound for the exhibition, are now full gifts, according to Robison.

"In each case, legal papers were drawn up to guarantee the gift at least by the time of death," he says. "Knowing the problems museums have had with promised gifts that finally didn't come, I was tough about that. There are many people willing to promise but not make a present gift, and they are not in this show. These are no mere cocktail promises.

"The news here," says Robison, "is not the number of things held back or given in part, but the number fully, presently given. Since the early days, there was a lot of concern. Could it be done? Times were bad, tax laws were changing for the worse. I was always optimistic -- I've never believed that tax laws made the difference. I set a goal of 100 donors and 200 works, and at last count we have 150 donors and 550 works."

Proof that taxes are not necessarily the bottom line, says Robison, became clear to him when European friends began offering gifts. "I always thought of this as an American affair," he says, "until two years ago, when I was in London having dinner with a friend, a collector-dealer, and started talking about the 50th. To my great surprise, he said he might like to make a gift. Next day, the very same thing happened, and a light bulb went on! There are many people around the world who love the National Gallery and the way we do things, and our standards." Since then, a dozen or so Europeans have given gifts, most of them old master drawings and prints.

"Another fascinating category is artists' gifts," says Robison, pointing out that, like European donors, contemporary American artists derive little or no tax benefit by giving to museums. "These two categories make clear that that people's generosity goes well beyond their tax benefits."

With gifts still rolling in, the National Gallery is prepared. "In the back cover of the exhibition catalogue we have a pouch for information about whatever comes in after the deadline," beams Brown. "We've already got some things to go in the pouch."