By now it's almost a tradition. When summit meetings are conducted between Mikhail Gorbachev and President Reagan, great paintings are exchanged.

In the latest such transaction, timed to coincide with the Soviet leader's arrival here today, the Soviets have sent us a late masterwork by Titian -- his disturbing "St. Sebastian" from the Hermitage in Leningrad. The seven-foot-high canvas will go on view this morning in a one-picture exhibition at the National Gallery of Art.

The exchange will be completed in 1989 when the gallery responds by sending to the Hermitage El Greco's "Laocoo n."

Is it mere coincidence that both of these great pictures seem to deal, if distantly, with military matters like those the two leaders will discuss?

The martyred St. Sebastian, analysts may note, has been successfully attacked by a flight of short-range missiles -- a quintet of Roman arrows.

Laocoo n, a Trojan priest, also died a martyr -- but to verification. Understandably suspicious of the giant wooden horse that appeared outside his city, he thrust a spear into its flank -- and was punished for his doubts when a pair of Neptune-dispatched serpents crushed him and his sons.

Both pictures were among the last completed by their painters. Titian's "St. Sebastian" had not yet left the master's Venice studio when the aging master died -- of the plague -- in 1576.

It is by any measure an extraordinary picture. Completed when Titian was well into his eighties, it has none of the precision, that cleanly ordered airlessness, of earlier Italian art. Of the great Italian masters, only Michelangelo in his last roughly chiseled statues handled surfaces so freely. When Titian made his "St. Sebastian," he applied its layered glazes not just with his brush, but with his fingers and his thumb. Its paint strokes seem to writhe; the dark sky appears to boil. It once was thought unfinished. During the 19th century, curators at the Hermitage were so unnerved by its turmoil that they took the picture from the walls and placed it in a storeroom. Today it is acknowledged as one of Titian's grandest works.

Is that form at lower left a fire or a rock? Is the passage just behind the saint a field or a sea? There is no way of telling. Save for its central figure -- whose left leg seems on fire -- the painting looks almost entirely abstract.

The "St. Sebastian" is the second superb Titian borrowed by the gallery -- for a single-picture show -- in the past two years. In January 1986, the gallery displayed a late Titian just as awesome, "The Flaying of Marsyas" from Kromeriz, Czechoslovakia. It, too, is a canvas drenched in suffering and grief. In both pictures, Titian could be responding to contemporary events.

Marsyas was skinned alive by the god Apollo. But only a few years before the picture was completed, a similar fate was visited on a Venetian hero, Marcantonio Bragadin, by a general of the Turks. Though Bragadin was killed, the Venetians won the war at the Battle of Lepanto. The Marsyas thus blends martyrdom with victory, redemption with defeat.

The story of St. Sebastian, an officer of the Roman imperial guard who was ordered shot with arrows when he declared his Christianity, allowed the pious Christian artist an opportunity to paint the male nude. Titian, however, appears to have had something else in mind.

Because the ancients believed that disease was spread by the arrows of Apollo, the martyred St. Sebastian, pierced with arrows for his faith, was thought to guard the prayerful from the depredations of the plague. When Titian was at work on his "St. Sebastian," that most dreaded of epidemics was devastating Venice. Perhaps a third of the Venetians died. One cannot escape the feeling that Titian, in this picture, was foreseeing his own death. The "St. Sebastian," like the "Marsyas," another essay in redemption, transmutes horror into art.

The U.S.-Soviet exchange was arranged a few months ago during a luncheon meeting here between J. Carter Brown, the gallery's director, and Soviet Ambassador Yuri Dubinin. Both men were at the gallery yesterday. "Such an exchange," said Dubinin, "once would have required months, perhaps years of negotiation." Dubinin added that when the picture arrived here a few days ago it was sent to his embassy, not the gallery. "With the painting at the embassy, without any guarantees, I passed a troubled night," he said.

The "St. Sebastian" was owned by the Barbarigo family of Venice from 1581 until it was purchased for the Hermitage in 1850. It has since left the country only once, for the Titian retrospective held in Venice in 1935. Though its departure date has not been set, Brown said he hopes it will remain on view -- in Gallery A, next to the West Garden Court of the West Building -- until Feb. 15.

El Greco's hallucinatory "Laocoo n" (which shows, instead of Troy, Toledo in the background) may well have been painted for the artist's pleasure. It does not seem a commissioned piece. Completed circa 1610 and discovered in the painter's studio following his death, it has been in Washington since 1946, when it was given to the gallery by Samuel H. Kress. Its 1989 visit to Leningrad will help celebrate the 225th anniversary of the Hermitage's founding.