Men of high propriety - Kenneth Clark, for instance - long have been discomforted by the sexy boudoir beauties of 18th-century France.

They are almost never serious, they almost always flirt. In sentimental painting after sentimental painting these amorous soft creatures flash their well-fleshed thighs. Of Europe's major pictures, Clark likes theirs the least. They are, he once said, too frothy, too lacking in high seriousness. But then he paused, and smiled "However," he continued, "I own a beautiful Fragonard."

Jean-Honore Fragonard was born in 1732 and died in 1806. For the French aristocrats he entertained and teased, it was the worst of times - the French king was about to fall, the guillotine was busy - but from the pictures that he left us it seems the artist hardly noticed.

He continued courting pleasure - the pleasures of the countryside, garden and bedroom, and, above all else, the pleasures he derived from the act of making art.

Two delicious shows of drawings - one of Fragonard's, one of Hubert Robert's (1733-1803), his friend and fellow student - go on view tomorrow in what is now called the West Building of the National Gallery of Art. Either show alone would repay a lengthy visit. Seen togeter, on the Gallery's first floor they present a feast for mind and eye that is greater than its parts.

The exquisite drawings breathe the sweetly scented soon to vanish spirit of the artists' time. In the last years of their lives both men helped to organize the new national art museum that would become the Louvre. At the start of their careers they both won appointments to the French Academy in Rome.

They fell in love, together, with Italy - its sunlight, its landscape and its past. In those days before photography, when one role of the artist was to function as recorder, they both made snapshot sketches, sometimes working on the same sheet, of the crumbling and picturesque ruins. Though Fragonard no doubt is the greatest artist, they both used ink and wash and chalk with wonderous facility. So familiar were their early styles, techniques and intentions that their Italian drawings have often been confused.

But if their pictures were similar, their personalities were not. It is the difference between them one remembers from the show.

Because his art is never grim, Fragonard is often seen as a one song artist whose only tune is frivolous. But he drew so fluidly in so many different styles that his show's huge variety undermines that myth.

Hubert Robert, in contrast, gets credit for deep seriousness because he filled his drawings with somber and convincing - though frequently invented - images of Rome's crumbled past. Weeds grow in his temples, ancient statues turn to dust and columns fall. He drew such scenes so often that he was known in his day as "Robert of the Ruins." Diderot, for one, thought such pictures "lofty." Everything vanishes, everything perishes, everything passes away," wrote Diderot, "the world alone remains, time alone continues." Because of the Ancien Regime was crumbling, too, Robert's views of ruins often have been regarded as politically significant.

But the viewer who begins the show thinking Robert is weighty while Fragonard is flighty leaves feeling that it's probably the other way around.

Robert had a gimmick which he repeated with great skill time and time again. He gave his clients what they wanted and was repaid in kind. Though his pictures, much like Andrew Wyeth's, were made to raise a sob, what he sought was fun. He lived on the fast track.

"Of all the artists I have known, Robert was the one most often encountered in society," wrote Mme. Vigee-Lebrun when she heard that he had died. "Incidentally, he enjoyed it very much. Fond of all pleasures, including that of good food, he was widely sought after, and I do not think he had dinner at home more than three times a year. Evenings at the theater, balls, concerts, meals, parties in the country . . . all the time he did not spend working was spent in entertainment."

Robert was a fashionable decorator of town and country houses. He was commissioned to redesign the gardens of Versailles. And though his Italian scenes might have been read as hymns to Republican virtues, nobody was fooled: He was imprisoned during the Revolution.

Fragonard, though his subjects were anathema to the new order, appears in retrospect to have been a revolutionary artist. He was not anchored to the past. He improvised as fluidly, and in as many private styles, as Picasso did later. He could improvise a scene with loosely swirling lines, or perfect a tightly finished drawing. No matter what he drew - a saucy bedroom scene, a nymph wrapped round with vapors, sunlight on a tree - his every mark astonishes us with its mastery, its ease.

These two shows are splendid, and their timing is just right. Robert knew Piranesi, and the ruins Robert shows us sharpen and extend our memories of the equally imaginary but far stranger ruins that we saw in the East Building's show of Piranesi's prints Fragonard, meanwhile, is the opposite of Edvard Munch. Fragonard loved women and everything about them. Munch, whose retrospective is now in the East Building, would look at them and scream.

"Drawing by Fragonard in North American Collections" was organized by Eunice Williams of Harvard's Fogg Museum. "Hubert Robert: Drawings and Watercolors" was organized by Victor Carlson of the Baltimore Museum of Art, who also wrote the first rate scholarly catalogue that accompanies the show. These twin exhibitions close Jan. 21.