One hardly knows what to expect of the Diego Rivera exhibition at the art museum here. Rivera's greatest achievements, after all, were the stupendous mural cycles that remain resolutely in place where he painted them, in Mexico and points north. What could it be, this huge show of the muralist without the murals?

The answer, it happens, is quite a lot. No question, the murals are sorely missed (though a 35-minute film helps fill the unavoidable gap). The sheer amount of the work he did on the walls of Mexico's major buildings and in the United States from 1922 to the mid-1930s is hard to fathom. More impressive still is the quality, especially of the Mexican murals. He filled vast architectural spaces with unforgettable images from Mexico's colorful, tragic history. Together with his great contemporary, Jose Clemente Orozco, he forged a national identity for his native land.

But Rivera's prodigious productivity -- it is said that he created some 3,000 oil paintings and about 25,000 drawings -- ensured there was much to choose from. What this well-selected retrospective, containing approximately 100 paintings and 140 drawings, does best is to trace the complex underpinnings of his art from the time he was a talented student at the Academy of San Carlos in Mexico City to 1922, the year the great mural episode began.

Rivera was born in Guanajuato, a provincial mining town, in 1886. He had a twin brother, who died a year later. Both of his parents were teachers. His father's liberal political views were inimical to the local establishment; as a result, the family moved to Mexico City in 1892. Diego was fondly called "the engineer" by his family because of his interest in things mechanical, and even as a child he drew a lot. Like Picasso, he was a prodigy. There is a quite respectable pencil portrait of his mother in this show, made when he was 10. Two years later, shortly after his enrollment in San Carlos Academy, Mexico's best art school, he demonstrated himself to be one of the preeminent student draftsmen, as one can plainly see in a number of works here.

The San Carlos training was thorough and rigid. Students copied drawings by "masters" and drew from plaster casts of classical statues; they mastered rules of perspective and composition. Rivera never forgot a thing he learned: This early education in academic fundamentals served him well more than 20 years later. While in school he was stimulated also to look carefully at the local landscape, so that by his 18th birthday he was able to paint superior, spacious vistas of village life in the Mexican highlands.

At the turn of the century a period of travel and study in Europe was customary, almost mandatory, for good Mexican art students. Rivera's lasted, with but one six-month interlude at home, for nearly 15 years: He departed at age 20 in early 1907 and returned, age 34, in the summer of 1921 -- which is to say he left as an advanced student from the hinterlands and came back a man of the world and an artist fully prepared for the unique challenge of the new Mexican mural program.

Rivera's European years -- culturally rich, financially the opposite -- are fully chronicled in this exhibition and catalogue. With his big, round, omnivorous eyes and his probing intellect, he devoured the art of the museums in Madrid, London and Paris. With his contentious, outgoing and irresistible personality he attracted brilliant friends and enemies in various Spanish, Parisian and expatriate circles. (He was a regular, for instance, at Henri Matisse's weekly gatherings during the war years; he had a big quarrel with Picasso; he and Amedeo Modigliani delighted in "scandalizing the cafe crowd of Montparnasse with their bizarre tales and behavior.") Despite his grotesque physique -- he weighed more than 300 pounds -- he drew women to him (he lived for much of the time in common-law marriage with Angeline Beloff, an e'migre' Russian artist, but there were others).

Artistically, his instincts were roaming. He went through a Symbolist phase, an El Greco phase, a Spanish modernista phase, a divisionist phase; he found a home, for about five years, in the synthetic Cubism of Juan Gris. In each of these styles Rivera created significant canvases. His large painting "The Old Ones" (1912) is a startling portrayal, at once realistic and hauntingly symbolic, of three ragged old men in burnt Spanish mountain country. Its counterpart of 1913, "Portrait of Adolfo Best Maugard" (a Mexican painter), is a similarly arresting combination of El Greco and modernist sources, in this case the dynamic urban visions of Robert Delaunay.

Rivera's Cubist masterwork, "Zapatista Landscape" (1915), referring of course to Emiliano Zapata and his fierce cadre of humble Indian fighters from the south, is a beautiful painting, and awesomely strange -- a melding of ice and fire. The exceptional harmony of its composition of interlocked flat elements and its calm color scheme of grays, whites, browns, blues and reds is undercut by the theme, featuring serape, sombrero and gun. The painting is like a noble flag. Behind it one can hear, at a chilling distance, the fearsome revolutionary cry -- "Death to the hacendados!" -- and can feel the bloody reality of the still raging Mexican civil war.

"Zapatista Landscape" is but one sign that while in Europe Rivera was in close touch with the tumultous events in his homeland. It is a sign, too, of his developing political stance. Rivera became a Communist. During the 1920s and 1930s he was attacked from both the left and right -- an anti-Stalinist, he was expelled from the party; he hosted Leon Trotsky for several years when the latter arrived in Mexico; he was vilified by conservatives in his country, and his capitalist patrons in the United States closed down the money gates when he refused to remove Lenin's portrait from his Radio City mural.

Rivera's growing stature as an artist did not go unnoticed in Mexico, and one of the people who did the noticing was Jose Vasconcelos, a brilliant liberal philosopher and scholar who became minister of education in the early 1920s. The civil war had ended, and to Vasconcelos fell the formidable task of creating an ambitious public eduction system. He was determined that, in addition to becoming literate, the masses of new pupils as well as the public at large were to be instructed in the history and values of the Revolution.

Rivera was his main man. In 1920, while still rector of the national university, Vasconcelos sent the artist on a 17-month tour of Italy to study the murals there -- Giotto, Ucello, Mantegna, Piero della Francesca, Michelangelo. Consequently, when Rivera returned to Mexico City, he came prepared to lead, and he did, often to the discomfiture and ire of his fellow muralists. He literally took over the entire program for the decoration of the new Ministry of Education, banishing the other artists from the premises and painting over most of their murals.

Herculean is the only apt word for Rivera's labors there. He thought nothing of 18-hour working days (and somehow had time left for as full a life as has been lived); over a period of five years he completed 116 fresco compositions in the courtyards, hallways and stairwells, each related to the generous theme, "A Cosmography of Modern Mexico." Together with his frescoed main stairway in the National Palace (1929-30), devoted to Mexican history "From the Conquest to the Future," they make up an encyclopedic record of the country's regional customs and a didactic, Marxist reinterpretation of its history. They are, of course, the benchmarks of Rivera's mature art.

Not much of this can be captured in a museum exhibition. Though there are exceptions, most of the paintings Rivera did in his broad, generalizing muralist style look somewhat adrift. Their meaning comes more from the context, the vast intellectual construct that was Rivera's Mexico, than from their intrinsic force. Not so the drawings. Rivera was a fabulous and unstoppable draftsman, and the many great examples of this gift breathe with immediacy and life, almost as if his hand had just left the paper. (His portraits and self-portraits, too, form a rewarding sub-theme in the exhibition.)

But by no means does the show skirt the mural issue entirely. There are sketches of varying degrees of finish for murals, paintings and two frescoes done after subjects in particular murals and, most resounding, a number of cartoons for the walls Rivera decorated at the Detroit Institute of Arts. These cartoons, full-scale drawings on paper used by the artist to transfer his images to the walls, depict allegorical female figures and monumental hands representing the black, red, white and yellow races. They excite by their very size and, more than anything else here give a strong sense of the boldness and sculptural simplicity of Rivera's draftsmanship when he was working on a grand scale.

Rivera often said that his art was "reborn" after he returned from Europe, and in a sense this is true. He discovered his great subject there -- Mexico itself. It was no coincidence that Rivera became a hugely enthusiastic and knowledgeable connoisseur-collector of Mexico's folk and pre-Columbian art, and this infused his own work with a broad-minded nationalistic passion that we have come to associate with the Mexican mural renaissance.

But the question of his mature style is not so simple. Though his art obviously gained a new clarity and vitality, Rivera clearly was synthesizing the lessons he had been accumulating since childhood. His murals are marked not only by the superb command of perspective he picked up as a schoolboy, but also, at their best, with the orderliness and simplicity of Giotto; nor could he have organized his often jam-packed historical scenes without a command of the juxtaposed planes of Cubism.

Art historians sometimes speak of a "magic moment" in art -- a place and time where intellectual and social forces come together in the presence of a number of supremely gifted artists to produce extraordinary results. Mexico in the 1920s was such a place and time. Rivera and Orozco, the all-consuming rationalist and the fiery expressionist, were the giants who pulled it all together. The Rivera exhibition, despite inevitable shortcomings, offers a rare chance to see and think about this almost overwhelming artist.

"Diego Rivera: A Retrospective," was organized by the Detroit Institute of Arts, where it originally went on view last February. It will travel to Mexico City, Madrid and Berlin after leaving the Philadelphia Museum of Art on Aug. 10.