The Soviets have lent the National Gallery their only Leonardo, and it is not a beauty.

"From Leonardo to Titian: Italian Renaissance Paintings From the Hermitage," the small, instructive show which goes on view today, has a crippled star.

Leonardo da Vinci's "Madonna and Child," also known as the Benois Madonna, is the most important - and the least attractive - of the 11 paintings on view.

The Virgin has been badly damaged. Her eyebrows, of example, were long ago removed by some ham-handed restorer. The landscape in the window, if there was a landscape, has been removed or overpainted; it is no longer there.

Kenneth Clark, a scholar who knows his Leonardos, has called the window "ugly" and the baby "monstrous." The surface of the painting, once smooth as enamel, has been flattened and abraded.

Yet despite its flaws, this cruelly treated picture dominates the show.

It is only 19 inches high. Legend has it that the painting was carried into Russia by a band of wandering Italian musicians. Just before the Revolution, it was purchased by the Czar for $1.5 million - making it, at that time, the most expensive painting ever sold.

Leonardo's oils are extremely rare. "Depending on whose attributions you accept," writes Carter Brown, the Gallery's director, "there are only somewhere between a dozen and 20 in the world."

It would be difficult to overstate the significance of this one. It is a revolutionary painting. When the 26-year-old Leonardo made it - 500 years ago - he produced a picture that did not resemble any made before.

"The Benois Madonna," writes Everett Fahy, the director of the Frick Collection, who wrote this exhibition's catalogue, "is not just another picture of the Virgin and Child - it is a turning point in the history of Western art."

The other paintings here - the Raphael (if it is a Raphael), the Melzi, the Palma Vecchio, the Correggio, even the Pontormo - sustain that contention. For most of the next century the Italian masters imitated, or adjusted, or rebelled against, the spirit that we still detect in the Benois Madonna.

Cima's "Annunciation," with which this show begins, suggests the old and strict conventions that Leonardo shattered.

Despite the trompe l'oeil insects he has added to his picture (they are there to suggest sin), Cima (1460,1517) was a conservative traditionalist. His Maddonna, dressed in red and blue, is static as a statue. The box-like room she occupies, its columns and its furniture, obey the rigid rules of single-point perspective.

Leonardo's work, in contrast, is a symphony of curves. "Leonardo observed," writes Fahy, "that the vital elements of a composition, its outlines and proportions, must resemble music in order to be beautiful. . . It's coloring reveals even more of Leonardo's genuis. Artists before him usually modeled forms in paler or darker values of the same color.

"For example," he continues, "a painter depicting a figure wearing a blue robe would use pale blue for the parts of the robe in strong light and deeper shades of blue for the parts in shadow. But this causes every element of the picture to appear separate from the next. . . as in a stained glass window. . . Leonardo overcame this difficulty by making the simple but scientific observation that color dispppears in darkness. . . He uses color only where light allows one to see it." The figures in his pictures are both idealized and true and nature, solid yet sublime.

Few painters, once they viewed his works, were able to ignore his sinuous compositions, and shadowed, melting colors, his idealizing spirit and technical inventions. Some followed them by rote. A work by one such artist, Francesco Melzi of Milan, is included in this show.

The two men were close friends. Melzi, though a wealthy man, long served Leonardo, his elder by some 30 years. He copied drawings for the master and took dictation in Leonardo's notebooks. When Leonardo died in 1519, he left Melzi his art books and materials.

Melzi was devoted to the older master; one sees that in his work. Melzi's "Flora" in this show is, despite its awkwardness, so Leonardesque in its murky light, its finish, even in its details - Flora's Mona Lisa-like smile, the ringlets of her hair - that the painting was, for centuries, thought to be a Leonardo. But, as Fahy notes, Melzi's brushwork is "laborious," the folds of Flora's dress are "monotonous," and her left arm has the look of a "disembodied limb."

Not all of Leonardo's students were as slavish as Melzi. Pontormo, whose Mannerist oil, "Holy Family With the Young St. John," is a highlight of this exhibition, was instead of rebel, even something of a loony.

"He never went to feasts or to other places where crowds collected for fear of being crushed, and he was solitary beyond belief," writes Vasari. "The room where he slept and sometimes worked was approached by a wooden ladder which he drew up after him."

His picture in this show, unlike Leonardo's, is less idealized than perverse, The Virgin, in her pink and yellow headdress, almost seems to float; the Christ Child and the young St. John both look a little mad.

Other painters here remained loyal to Leonardo's example. Palma Vecchio's "Portrait of a Young Man Holding a Glove" demonstrates that debt in its subtle, quiet colors. Like Correggio's beautiful "Portrait of Ginerva Rangone" (she holds a silver dish of opium), it shows how far into their own worlds the finest 16th-century painters were able to proceed even as they followed Leonardo's lead.

The Hermitage exhibit includes a splendid double portrait by Lorenzo Lotto and a peculiar little Mannerist picture, done on slate by Primaticcio, that is, in its own way, as weird as the Pontormo. The show is capped by an undisputed masterpiece by Titian, the Repentent Magdalen of 1560.

No other picture here is as freely painted. In its every passage, in her robe and hair, her open book and redrimmed eyes, one sees the inimitable mastery of his brush - of of his fingers. Titian did not try for Leonardo's flawless finish. The Magdalen's glass vial is as freely summarized and as loosely painted as the glass decanter in the Phillips Collection's large Renoir.

The collection of the Hermitage is stronger by far in Dutch, French and Flemish pictures than it is in works from Italy. If Dr. Armand Hammer keeps arranging loan shows from the Soviets - and he seems indefatigable - it is mouthwatering to imagine the borrowings from Russia that may yet be in store.

The Armand Hammer Foundation, which last month gave more than $1 million to the Corcoran Gallery of Art, also paid for the exhibition, which will travel to Los Angeles and Manhattan after closing here on June 24. CAPTION: Picture 1, Da Vinci's "Madonna and Child,"; Picture 2, Melzi's "Flora,"; Picture 3, Pontormo's "Holy Family With the Young St. John."