hen Henri Matisse, who started painting late, first picked up his brushes at the age of 20, he felt himself "transported into a kind of paradise." He was already in his forties when, in 1912, he first visited Morocco and discovered in that alien land an otherworldly Eden that forever after gripped his colored dreams.

The paintings he produced there are decorative, difficult, eerily disturbing. Looking at them now is like squinting into sunlight. It is as if he were painting the inside of his eyelids, capturing on canvas the paradise that flickered halfway between the imagined and the seen.

"Matisse in Morocco: The Paintings and Drawings 1912-1913," the joint project of the U.S.A. and the U.S.S.R. that goes on view today at the National Gallery of Art, is the smallest -- and the weirdest -- of the three Matisse exhibits held there in the past 13 years. Not everyone will love it. It may well be a bit too rough, a bit too free, to catch the public's pleasure, for its luxuriance is countered by a quick and restless searching. There were 170 pictures, calmer works by far, in "Henri Matisse: The Early Years in Nice 1916-1930," which opened at the gallery in 1986. Here there are but 20. There were more than 200 late collages in the gorgeous paper cutouts show of 1977. Those old man's floating reveries of fronds and drifting nudes felt wholeheartedly invented. But these paintings of Morocco are still depictions of the actual. They are records of Tangier.

They are tethered to its costumes, its casbah and lush gardens, its hashish fumes, its warriors, its cafes and its terraces, its narrow cobbled passageways, its white Islamic domes. But tethered may be too strong a word. Matisse, who went there twice, in 1912 and 1913, employed the city as a charm, or perhaps a kind of drug, to propel him through the real. "I went to Tangier," he explained, "because it was Africa." His art bounced off its otherness. Matisse employed its strangeness -- its kif pipes and its doe-eyed whores, its foliage, its shadows -- as a kind of mental trampoline to spring him into a new realm where space was built ecstatically of planes of colored light.

Since Napoleon conquered Egypt, adventurous French painters -- Delacroix, most notably -- had traveled to North Africa, to what they called the Orient, in search of the exotic -- scimitars and Sultans, naked maids in tiled pools, the lions of the desert, fine Arabian horses prancing on the strand. But Matisse in his Moroccan days, that last great Orientalist, had something else in mind.

In 1912, in Paris, painting was dissolving. Picasso and Braque, forging past Cezanne, were splintering the real, shattering the grayness of newspapers and pipe smoke into glinting cubist planes. Marcel Duchamp was cogitating images of transcendental speed. A vision we call modern, an art that broke away from the toned and measured look of 19th-century painting, was everywhere emerging. The Matisse of 1905, the fauve, the "wild beast," had forced upon the world ferocious colors never seen before. He had struggled to surpass the hewn structures of Cezanne, and the scientific hues of the post-impressionists. But now he was confronted by what seemed to him the void of the wholly nonobjective. He could not bear its emptiness. Oriental rugs, the miniatures of Persia, the icons he had seen on his voyages to Moscow, and carvings made in Africa, seemed to hint at a solution. But it was not until Morocco that his liberated color was reattached to life.

Perhaps the strangest thing about him is his tenuous and distant reliance on the real. Or on aspects of the real. His art was never social, it was wholly apolitical. Two world wars and the Depression left no mark on his pictures. Much about his art feels completely artificial. The models he employed in Nice -- those patient, hired women in their see-through pants lounging among rugs and screens -- strike us now as faceless props, anonymous, lethargic, only partially alive. If he lusted for their bodies, or paid attention to their minds, you'd never guess it from his art. Peculiarly unsexy, they have no personality at all. Yet he could not do without them. He was lost without the models that he once described as "the burning center of my energy." He needed them to paint.

He needed Tangier too. But not for its bright subjects, nor for its fresh details. He might easily have painted irises in Paris. He could have hired models and dressed them in djellabas without ever leaving town. The Orient, he said, brought him "revelation." Tangier set him free.

It is a city like no other. If you leave from Algeciras, and cross into Gibraltar, and then take the easy ferry ride across the narrow strait, you feel as if the distance from Spain to cozy England deep into North Africa has somehow been compressed into an afternoon.

The casbah's streets are carless still. The women are still veiled. Morocco, when Matisse was there, must have felt yet stranger, and considerably more dangerous, at least for Europeans. Such visitors from Britain as Sir Harry McLean of Scotland and Walter Harris of the London Times had only recently been captured, and held for ransom, in the Rif. In Fez, in 1912, the European population was massacred en masse. Did the menace of the place somehow spur his art?

You would not think so from these pictures. Tangier, that free port, has long served as a lure for those seeking the illicit. Sailors from Gibraltar have gone there for the whores, hippies by the thousands have gone there for the kif, and male homosexuals, following in the footsteps of Tennessee Williams, William Burroughs, Paul Bowles and the rest, have gone there for the boys.

But Matisse, the good bourgeois, seems hardly to have noticed Tangier's illegal charms. One amusing drawing here shows him sketching on a casbah street among veiled women, his propriety inviolate, in his swallow-tailed coat. It is true he hired Zorah, a beautiful young prostitute who dared show him her face, but when he sought her in her brothel, he took along his wife. Neither flesh nor drugs nor danger brought him to Tangier. What lured him was the light.

Tangier is known as "the white city." Its whitewashed domes and walls are bathed in blazing sunlight. But one of the first strangenesses encountered in this show is how little of that whiteness is recorded in his art.

The Tangier of Matisse is not white at all. It is a city ruled by blues. The gleaming domes, the minarets, the garden paths, the trees seen from his hotel room, and the shadows of "The Casbah Gate" are all washed with cooled-down greens and blues. If Matisse had a blue period, it was his period in Tangier.

The first great work one sees here, the so-called "Moroccan Triptych" borrowed from the Pushkin, is a hymn to varied blues. In the central painting, Zorah, the young model, poses on the terrace of her Tangier brothel, hovering in coolness as if floating in some antidote to Morocco's pounding heat. Nothing in this picture, save the kneeling model and the goldfish bowl beside her, is solid or substantial. The shadowed space around her (an astonishing prediction of the "Ocean Park" abstractions of California's Richard Diebenkorn) feels somehow manufactured of sheets of blue-green scumbled light.

Her mouth is but a smudge, her hands are hardly there, and yet we know from X-rays how assiduously the painter labored on her image. A still-surprising sketchiness, a refusal of high finish, lends a shivering of quickness to even the most carefully considered pictures in this show.

Of his three garden paintings, one -- the National Gallery's "The Palm" -- was painted all at once in what he later called "a burst of spontaneous creation -- like a flame." A related picture near it -- Stockholm's wonderful "Acanthus" -- sings of the same suddenness, though he took it back to Paris, then returned to Tangier and pondered it for months.

Almost all these pictures glow with contradictions. In most of them one feels the reality of Tangier transmuting into dream, and a hungering for newness that nonetheless seems rooted in the past. Time feels shaken too. The pictures on display -- all the greatest ones -- simultaneously suggest sketchiness and timelessness, as if years of preparation had been distilled into speed.

By far the grandest painting here is that "souvenir" of Tangier from the Museum of Modern Art known as "The Moroccans." Dated 1915-1916, it is the latest in the show. It seems organized in chapters. At upper left one reads a white-domed tomb, a terrace and a terra cotta pot of blue-and-white-striped flowers. The round forms grouped beneath them are intentionally ambiguous: Some will tend to see them as ripe yellow melons and the green leaves of their plants growing on a trellis; others may well read them as Moslems at a mosque, kneeling on a tiled floor, bowing down in prayer.

The right third of the painting, like some fleeting dream, cannot be deciphered. A blue-caped turbaned figure sits there, but nothing else is clear. The forms that float above him might be shadowed archways, or glimpses through a window, or man in a burnoose sleeping by a table, but there is no way to be sure. This large, majestic canvas is a construct of memories, of memories transformed. Perhaps its strangest transformation is that the white light of Tangier has here become its opposite. That light of "The Moroccans" is a light composed of blacks.

"Matisse in Morocco" is a kind of reunion. Nine of its best pictures, purchased from the painter by two Russian businessmen, Sergei Ivanovich Shchukin and Ivan Abramovich Morosov, remain in Soviet collections. Most of the remainder are scattered through the West. Five experienced scholars, Moscow's Marina Bessonova, Washington's Jack Cowart, Manhattan's John Elderfield, Leningrad's Albert Kostenevich, and Pierre Schneider of Paris, and the staffs of four museums -- the National Gallery, the Museum of Modern Art, the State Pushkin Museum in Moscow, and the State Hermitage in Leningrad -- organized the show.

Together they've produced an exhibition catalogue nearly 300 pages long. Elderfield and Schneider have thought deeply on the paintings, and Cowart and his colleagues have painstakingly unearthed 67 Matisse drawings, all except a dozen previously unknown. There are 45 on view.

Most are little more than jottings, and though the jottings of Matisse are not to be discounted, do not go expecting to find an exhibition filled entirely with masterworks. Two of its sketched paintings -- "Zorah Seated" and "Moroccan Woman," both from private collections -- are minor things at best. "Moroccan Woman," truth be told, is a coarse and second-guessed and pretty ugly work of art.

Matisse is in transition here. Shimmering around this tightly focused show is an experimental edginess not at all apparent in his patterned Nice interiors or in his calm and radiant cut-outs. But all those later works of art owe something to the brightness, the space-defining colored light, the foreignness, the luxury, the swooping arabesques -- to that Oriental garden of painterly delights -- he first discovered in Tangier.

"Matisse in Morocco" will travel to New York, then to Moscow and to Leningrad (where it will be joined by the grand "Moroccan Cafe'," a summarizing canvas too delicate to tour) after closing here June 3.