"Treasures of Ancient Nigeria" -- the major loan exhibit whose astonishing bronze sculptures have already altered the Western world's conception of Africa's art history -- will open here Dec. 18 at the Corcoran Gallery of Art, and will run through Jan. 31.

"Without question, this is the most important exhibition of African material ever sent to the United States," said Warren Robbins, director of the Smithsonian Institution's Museum of African Art, whose federal museum, in an unusual arrangement, is serving as co-sponsor of the Corcoran's display.

Its subtle metal portraits are more peaceful than demonic, more lifelike than abstract. They will seem, to Western eyes at least, to have more in common with classical Greek sculpture than with the harsher wooden masks for which the tribal arts of Africa are most widely known. Because objects made of cowrie shells, fabric, feathers and carved wood decay very quickly in the sub-Saharan climate, most of the art of Africa that has survived is relatively new. But these Nigerian works were made of cast bronze and cast copper, of stone and terracotta -- and the oldest of them date from the 5th century B.C.

The Nigerian exhibition was organized by the Detroit Institute of Arts, and opened there in January.

Though Robbins, Smithsonian Secretary S. Dillon Ripley and Joan Mondale lobbied for a showing at the Museum of African Art, the Nigerians, understandably, found the awkward exhibition spaces of that small museum unworthy of their show. Instead, it will be seen in the skylit galleries on the Corcoran's second floor.

Sculpture from seven separate African civilizations will be included in the exhibition. The oldest of these is the culture known as Nok -- named for the northeastern Nigerian village in which a clerk at a local tin miner found a terracotta head in 1943. (The clerk, not realizing the rarity of the object, at first used it as a scarecrow in his yam field.) Since then more than 150 other Nok artifacts have been unearthed. The Nok culture flourished between 500 B.C. and 200 A.D.

The other civilization represented include those known as the Igbo-Ukwu, the Ife, the Owo and the Benin. A set of Esie soapstone figures and Ikom monoliths will also be displayed.

The Benin king, or oba, was the only member of his society allowed to own bronzes of the sort included in this show. When his palace -- and his way of life -- was destroyed by British soldiers in 1897, thousands of his bronzes were taken back to Europe, where they were eventually acquired by various art museums. His was the only old Nigerian civilization known widely in the West prior to the tour of the current show. So fine were those pieces, so delicate their casting, that ethnocentric Europeans for many years suspected they must have been made by Romans, Greeks or other whites who somehow worked in Africa. That argument, however, is easily demolished by the many other similar, though much older, objects to be shown in "Treasures of Ancient Nigeria."