One evening, in the midst of the avalanche of honors that followed her 1970 retrospective at the Whitney Museum, Georgia O'Keeffe stood by the window of her adobe house in northern New Mexico and, out of the blue, said to a friend, "I still don't understand what all the fuss is about. I'm just up here doing my work."

O'Keeffe, a pioneering American modernist, would have been 100 on Nov. 15 -- and probably still wondering -- had her life not ebbed out at 98. In celebration of that extraordinary life and career, the National Gallery today opens the first major O'Keeffe show in 18 years. It offers a whole new generation a chance to see what the fuss continues to be about.

This is not a show that shouts. Nor does it strive to be a definitive retrospective. Instead, it is a selective survey of 120 works -- one-third of them previously locked away in her estate -- intended to highlight areas in which O'Keeffe made her most original contributions, as perceived by curators Jack Cowart and Juan Hamilton.

The result: a purifying, clarifying exhibition that begins the process of tearing down 70 years of musty mythology by letting the best work speak for itself. The artist's voice has been powerfully amplified here with the publication of 125 of O'Keeffe's own letters, brilliantly edited and annotated by Sarah Greenough in the exhibition catalogue. Despite omissions as well as inclusions of lesser works that hint at commerce and compromise, this enterprise offers rare access, at last, to the authentic O'Keeffe -- the artist and the woman.

There is a sense of revelation about the show. For though O'Keeffe is arguably the best-known woman artist in American history after Mary Cassatt, some know her only as a painter of the the New Mexico desert, others -- older perhaps -- as a painter of giant, close-up flowers with erotic overtones. Few know the full range and variety of her work, since there has been no opportunity to take a true measure of her accomplishment for nearly two decades.

Some will doubtless find a flower too sweet,or an arching abstraction too heavily perfumed with color, or a painting of a cow pelvis against a garish background too posterlike and contrived. But there is so much here -- the early works, the spectacular New York skyscrapers, the meltingly beautiful flower abstractions, the sheer gutsiness and longevity of the career -- that even critic Clement Greenberg might be moved to soften his pronouncement 45 years ago that O'Keeffe's work amounted to little more than "tinted photography."

Nature was always O'Keeffe's touchstone, art her passion, from the days of her youth on a dairy farm in Sun Prairie, Wis. The second of seven children, she was the daughter of an Irish Catholic father and a Hungarian/Dutch Protestant mother who set great store by education. She began art lessons at age 11, before going off to boarding school in Virginia (now Chatham Hall). After studies at the Art Institute of Chicago, the University of Virginia, the Art Students League (under William Merritt Chase, also showing at the National Gallery now) and Columbia University Teacher's College in New York, O'Keeffe began teaching art in schools in Virginia, South Carolina and, finally, Texas, where her ecstatic relationship with the Southwestern landscape began.

It never ended: After 11 years in New York, following her discovery by photographer and avant-garde dealer Alfred Stieglitz, whom she subsequently married, O'Keeffe began regular visits to New Mexico in 1929, and settled there for good after Stieglitz's death in 1946.

"I've been all over the world," she said in her later years, "and nothing is as good as this."

The exhibition reveals itself quietly, slowly, beginning where her success began -- with a group of now legendary abstract charcoal drawings, known as "Specials," that marked O'Keeffe's artistic breakthrough in 1915. Working from shapes she said she saw in her head, but had never thought of using as art, she set out to convey through them the expressive power of music.

From South Carolina, where she was teaching at the time, O'Keeffe sent a roll of these drawings to her New York school chum and correspondent Anita Pollitzer, with whom she had often shared dreams of gaining Stieglitz's approval. Sensing that O'Keeffe, then 28, was ready, Pollitzer took the drawings to Stieglitz. The story of his subsequent passion not only for the art but also for the artist herself has been at the heart of both their biographies and -- since O'Keeffe's death -- has generated at least four more book contracts, one optioned to Steven Spielberg. Stieglitz -- married and 23 years older than O'Keeffe -- left his wife and moved in with O'Keeffe in 1918. They were married six years later.

The show opens with the very drawings that won Steiglitz, among them one particularly fine example, "Special, No. 13," which contains a lexicon of nature-based organic forms that O'Keeffe would continue to use throughout her career -- budlike, inverted raindrop shapes, lightning bolts, billowing clouds and riverlike flows of wavy lines. These very forms are at the heart of the large, dramatic "Black Place III," a semiabstract New Mexico landscape painted nearly 30 years later.

One of the big surprises in this show is a group of rarely seen small abstract oils titled "Series I," made just after O'Keeffe had accepted Stieglitz's offer of a year in New York to paint. Very like the charcoal "Specials" in form, they are, however, brilliantly colored variations that seem as fresh today as the day they were painted.

But in those early years, as well as later on, O'Keeffe constantly moved back and forth from figuration to abstraction, exploring the territory in between. She often worked in series, in which she distilled the landscape, mountains, sky, sunsets and flowers she saw -- along with her sensations about them -- into highly reductive but still expressive images. Several pairs of vibrant Texas watercolors dating from 1917 make the point, among them "Evening Star" -- nothing more than a few luscious, concentric curls of color -- and "Starlight Night," a simple grid of rich, deep blue over a long horizon.

In these small works that fill the first three galleries -- in essence, a mini-retrospective of works on paper from 1915 to the '50s -- can be found the seeds (and sometimes the fruit) of ideas O'Keeffe carried out on a larger scale in oil further on in the show. The smoky pastel titled "Dark Iris No. 3," for example, is a quivering distillation of the large, lavish "Black Iris III" painted one year earlier.

And "Red Hills and White Flower, 1937" -- distant hills dwarfed by a levitating foreground flower -- reflects a compositional device O'Keeffe used often in larger paintings with floating foreground skulls and bones, though rarely more effectively than in this haunting little pastel. There is throughout a remarkable ability to skew scale and make small images look vast. In his catalogue essay, Cowart states that O'Keeffe's small works are among her best, and he makes his case.

There are, however, some curious anthropomorphic overtones in these works: a snail shell curled against a red hill, for example, looks very much like a sleeping baby; and a series of clam shells seen closed, slightly open and open from the side, look like hooded figures confronting each other. Possibly Cowart and Hamilton, in what seems a bend-over-backward attempt to omit flower paintings that make more obvious allusions to sexual anatomy, have inadvertently made the point that O'Keeffe often saw human attributes in all inanimate objects, and not only sexual attributes in flowers.

Among the large-scale flower paintings -- a weak group overall -- the ravishing, salmon-pink "Flower Abstraction" from the Whitney shows O'Keeffe at her finest. Three of the "Jack-in-the-Pulpit" paintings willed by O'Keeffe to the National Gallery provide the most graphic illustration of how she progressively reduced and transformed a literal image of a single flower into total abstraction. In each of these images, she moves in closer and closer, concentrating finally only on the dark innards and on the Jack itself.

By 1929, O'Keeffe had reached a turning point in her life, and -- oppressed by New York and in search of new subjects -- she made her first visit to New Mexico, where her paintings took on an entirely new set of subjects, from the red hills and dark mesas to the stones and bleached animal bones that she found on her long walks and painting forays into the desert near Ghost Ranch.

There were some beautiful paintings from this period, mostly landscapes such as "Grey Hills" and the aforementioned "Black Place III," a semiabstracted view of a river cutting through mountains like a lightning bolt. After the paintings from the mid-'40s, however, the show fizzles, with one spectacular exception: the 24-foot-wide "Sky Above Clouds," inspired by O'Keeffe's frequent travels around the world. A great coda to her remarkable life, she painted it over a four- or five-month period in 1965, working dawn to dusk with the help of a sheepherder who assisted in stretching the canvas and mixing paint. She was 78.

Three years later, she lost the central vision in one eye, and for all practical purposes stopped painting, though a new assistant, young potter Juan Hamilton, encouraged her to go on working in various media. Hamilton, who cared for O'Keeffe during the last 13 years of her life, has written a charming, homespun memoir of those years for the catalogue, his first published statement. It was Hamilton whose share of O'Keeffe's record-shattering $79 million estate was recently reduced from roughly 70 percent to just over 10 percent by settlement with relatives who challenged O'Keeffe's will.

There are important paintings missing from this show, some requested but denied -- notably the outstanding flower paintings owned by O'Keeffe's late sister, Anita Young, whose charitable foundation refused to lend, citing bad feelings between Young and curator Hamilton. Instead, 10 of Young's paintings are scheduled to be auctioned off in New York in December -- the ultimate revenge from the grave. A painting given by O'Keeffe to Frank Lloyd Wright from the "Pelvis" series, which she considered her finest work of this type, was also refused because it, too, will go to auction next month to benefit the Frank Lloyd Wright Foundation. So much for charity.

Though O'Keeffe's letter-writing continued, it dwindled considerably after friends prevailed on her to install a phone in her home in the late '50s. Given that, there is amusing irony in the fact that the Southwestern Bell Foundation has underwritten this show, which continues at the National Gallery East Building through Feb. 21. It will subsequently travel to the Art Institute of Chicago (March 5 through June 19, 1988), the Dallas Museum of Art (July 31 to Oct. 16, 1988), and the Metropolitan Museum of Art (Nov. 19 to Feb. 5, 1989).