The Soviet Union has agreed to send to Washington its only painting by Leonardo da Vinci. It will be the first Leonardo lent to the United States since the Louvre sent the Mona Lisa in 1963.

The "Madonna With a flower," also known as the benois madonna, is the only unquestionably authentic picture by the italian master in Russia's state collections. Accompanied by 10 other Italian Renaissance pictures from the Hermitage in Leningrad-among them works by Raphael, Titian and Correggio-it will go on view here May 13 at the National Gallery of Art.

European governments, and individuals as well, are sending old master paintings to **washington with increasing frequency. A press confrence announcing the fall opening here of old master paintijngs from the Swiss collection of Baron Thyssen-Bronemisza will be held at the Gallery Tomorrow.

The name of Leonardo da Vinci is so exalted, and his pictures are so rare (fewer than 20 exit), that the monetary value of the Hermitage Madonna can only be guessed at. The National Gallery's "Ginevra de Benci," which, although much smaller, cost $6 million is the only Leonardo in a collection outside Europe.

Arrangements for the loan were first discussed last August when party chairman Leonid Brezhnev met in Yalta with Dr. Armand Hammer, the American billionaire.

Hammer, the head of Occidental Petroleum, has, since Lenin's day, maintained close relations with the leadership of Russia. Twice before, in 1973 and 1975, he helped arrange loans-of paintings by Matisse and 17th-century masters - between the Hermitage and the National Gallery of Art.

"I told Brezhnev," Hammer said yesterday, "it would be pretty hard to find an encore. He said, 'What do you want?' I told him, 'The Leonardo.'

"The Russians," Hammer continued, "are eager to show their good will to the United States."

"From Leonardo to Titian: Italian Renaissance Paintings From the Hermitage Museum, Leningrad" will visit Knoedler's, Hammer's private New York gallery, the Detroit Institute of Fine Arts, and the Los Angeles County Museum after closing here on July 29.

In exchange 17 Italian pictures from the collections of those institutions will be loaned to the U.S.S.R. Four of these-Titian's "Venues Binding the Eyes of Cupid," Moroni's "Titian's Schoolmaster," Bellini's "Orpheus," and Dosso's "Circe and her Lovers"-will be provided by the National Gallery.

The Gallery's Leonardo is not among them. "Well, it's different," said Director J. Carter Brown. "Under our bylaws no paintings on wooden panels are permitted to leave the building. The wood behind the Benois Madonna was shaved away in the 19th century. The painting is on canvas now, and can travel safely."

"We've had our eye on the Leonardo from the beginning," Brown continued. "It was on every want list. Everytime we asked, they said, 'Nyet, Nyet, Nyet.'"

"The Benois Madonna," says gallery Curator David A. Brown, "is the first painting by Leonardo in which he is entirely himself. Its composition is remarkably original. Look at those diagonals and cross-axes. So is what I call the picture's 'psycho-drama.' Follow the mother's glance from her joyous face to her enraptured child. Who else could have painted a baby that is both so life-like and so wise?"

The Benois Madonna was painted in 1478 when Leonardo was 26. Kenneth Clark describes it as a picture of "unquestionable uthenticity." Leonardo, he writes, was "determined to work out every detail according to his own standard of perfection, a standard which included scientific accuracy, pictorial logic, and finish . . . We must think of it as a changeling from the high **renaissance, an immature sample of the intellectual, classical style which Leonardo first came to public attention in 1913 when Mme. A. Benois, the wife of a Russian nobleman, offered it for sale to dealer Joseph duveen. As S. N. Behrman tells the story, "Getting wind of a new Da Vinvi was like discovering a new planet; Duveen was aquiver. . . He invited the Russian lady to come to Paris . . . Duveen took her and the painting upstairs to a room where there was sunlight and a small man with a magnifying glass. The man was (Bernard) Berenson-a fact Duveen did not mention to the lady. Berenson looked at the picture . . . He finally put it down and gave Duveen the high sign. It was indeed a Da Vinci. . . Duveen invited its owner to discuss a deal. She named the highest price ever asked for any picture in the history of art-$1.5 million."

Though Duveen agreed to the price, Mme. Benois reminded him that, under Russian law, she first was obligated to offer it to the Czar. She did, and much to Duveen's disappointment, the Russian ruler met her price.

"A clever lady," Hammer said. "She got Berenson to authenticate her Leonardo for nothing."

"Berenson," said David Brown, "wrote unflatteringly about the painting. He said the Virgin's mouth was

Kenneth Clark, too, has qualms about the painting. Its condition, he has written, is "bad. Parts of the composition can never have been happy: the baby was always monstrous and the drapery of the sleeve labored . . . The landscape seen through a window is absent from the Benois Madonna. The window itself, lacking a central transom, is ugly enough, and without a landscape it is really painful," he adds.

The 10 paintings from the Hermitage that will accompany the Leonardo include "Portrait of a Lady" by Corregio, Francesco Melzi's "Flora," Andrea del Sarto's "Madonna and Child," and "Saints Catherine, Elizabeth and John the Baptist," Cima da Conegliano's "The Annuciation," Primaticcio's "Holy Family With Saint Elizabeth," Titian's "Repentant Magdalene," Pontormo's "Madonna and Child," Lorenzo Lotto's "Double Portrait," and Raphael's "Holy Family." They will be displayed in the West Building among the Italian Renaissance pictures in the Gallery's collection.