SHE STANDS as a Colossus on tiptoes.

She was Isabel, the sculptor's obsession, a gargantuan bronze fertility goddess with generous breasts and hips, and gracefully curved arms and legs that seem to end in points.

At the Portrait Gallery, "Gaston Lachaise: Portrait Sculpture," shows Isabel in polished bronze, gleaming alabaster or leaden marble, a protean woman, bigger than life or small as a Barbie doll.

In 1915, Lachaise wrote to Isabel Nagle, a woman not yet his wife but already his inspiration, who was married at the time to a Boston businessman: "I think you are going to be happy with . . . the large figure. I want to create a miracle with it . . . as great, as you." After a chance encounter in Paris in 1902, he had followed her to America within three years. He married her after her divorce in 1917.

He was a man who loved women, loved the female form, not as something remote and generalized but with specific personality. Beauty has a bump on the bridge of her nose. But she's full of lusty grandeur.

"You are the Goddess I am searching to express in all things," he wrote to Isabel.

The sculptor's vocabulary was unlimited. He made a living for his wife and stepson Edward Nagle on portrait commissions. Through Nagle, he met E.E. Cummings, later sculpting a bust of him. There were so many great people with great faces to sculpt -- Georgia O'Keeffe, a smooth alabaster Nefertiti with her hair in a bun; and critic Henry McBride, face gouged from clay and then made into bronze, who had professed certainty that his was a face no artist could love; and poet Marianne Moore, photographer Alfred Steiglitz, painter John Marin, novelists, composers, conductors, doctors, and those who were, simply, patrons.

Viewed from the front and side, Lachaise's portraits show the poverty of two-dimensional art. His profiles are just as lively and perceptive as the frontal views.

A few years before his death from tuberculosis in 1935 at age 53, Lachaise began doing full-body portraits on commission.

"There are numerous barriers against such portraiture today, primarily in the minds of his subjects," patron Lincoln Kirstein wrote in a Lachaise exhibition catalogue in 1935. "But the Romans and the Egyptians were not dismayed by this frankness, and gradually we are becoming less so." Kirstein is the subject of "Man Walking," a balletic bronze nude. (But he didn't want his father to know.)

Degas had his ballerina; Lachaise had his nude baseball player, his nude tennis player, both perfect Adonises. Majestic, but not exultant.

"The bottom of it," Lachaise wrote to Isabel, "is that I do not know and love the male figure as I do the woman."

GASTON LACHAISE: PORTRAIT SCULPTURE -- At the National Portrait Gallery through February 16, 1986.