Rare Art Treasures on Loan From Siena. Exquisite new show at the Met in New York features individualistic, little-known art from the Renaissance. In paintings like these, the viewer is confronted by an approach to art that is, in certain ways, remarkably `modern' in spirit and execution. One might even go so far as to say that anyone delighting in the works of Klee, Morandi, or Magritte would be equally delighted by these Sienese paintings.

THE Metropolitan Museum here has pulled a fast one: With very little fanfare, it has mounted an absolutely first-rate exhibition that easily matches its current, heavily publicized Degas show and its recent assemblage of Paul Klee paintings, watercolors, and drawings. ``Painting in Renaissance Siena: 1420-1500'' is a magnificent collection of roughly 140 delicately drawn, exquisitely colored, and mostly smallish paintings and illuminated manuscripts. It is beautifully displayed and ideally situated in the museum's Robert Lehman Wing. It is accompanied by an excellent, lavishly illustrated 386-page catalog that includes illuminating essays on Sienese painting and patronage, as well as 100 color reproductions and more than 100 comparative illustrations. Of particular interest is the fact that no comparable exhibition of Sienese art has taken place outside Siena since 1904 and that many of the narrative cycles - small panels grouped together to tell a story or to develop a theme - have been reunited for the first time in generations, and, quite possibly, for the last time ever. Dispersed when the altarpieces to which they belonged were dismantled in the late 18th and early 19th centuries, these panels now belong to museums around the world, and are unlikely to be loaned again in our lifetimes.

Among the artists represented are Sassetta, his great but anonymous pupil, the Master of the Osservanza, Giovanni di Paolo, Mateo di Giovanni, Benvenuto di Giovanni, Girolamo da Cremona, and Liberale da Verona.

If these artists (with the possible exception of Sassetta) are unknown to most American viewers, it's probably because they've often been dismissed by art historians as minor figures having little to do with the grand sweep of Italian Renaissance painting as it moved majestically from Giotto through Masaccio to Michelangelo. And indeed, there is a quality of independence, even of idiosyncrasy, in Sienese painting of the 15th century, which sets it apart from most other Renaissance art.

Things were very different in earlier years, however. During the 14th century, Siena produced some of the greatest figures in European art - Duccio, Simone Martini, Ambrogio and Pietro Lorenzetti - but this golden age ended in 1348, when the Black Death swept through the city, killing roughly 60 percent of its population and helping to reduce it to a secondary power in the Italian peninsula.

Despite everything, however, Siena rebounded to produce, if not another golden age, then certainly a silver one of remarkably original achievements.

It is this second, less well-known period of Sienese creativity that this exhibition explores and documents. One of 15th-century Sienese painting's most intriguing characteristics was the rejection by its major figures of many of the basic tenets of Florentine art we now perceive as central to Renaissance theory and practice. By rejecting the more rational basis of Florentine painting, most particularly its insistence on one-point perspective, a careful and detailed study of nature, and a dependence on classical literature and philosophy, Sienese artists brought a uniquely individualistic and delightfully subjective note to their art.

This is especially noticeable in their frequently unconventional, even eccentric compositional schemes, novel expressive and story-telling devices, and sumptuous coloristic effects achieved, as often as not, by glazing over gold or silver surfaces with rich but sensitively applied strokes of color. It can be detected in the seemingly primitive but concise and consistent linearism of di Paolo's charming ``The Creation and Expulsion of Adam and Eve From Paradise;'' the theatrical, three-part composition of ``The Burial of St. Monica and St. Augustine Departing for Africa'' by the Master of the Osservanza; and the stark depiction of a desert in that same artist's ``Saint Anthony Tempted by a Heap of Gold.''

This sharply focused and charmingly innovative creative spirit also manifests itself in the refined draftsmanship and delightfully rendered human touches of di Giovanni's ``The Way to Calvary'' and ``The Vision of St. Augustine,'' and in the Master of Osservanza's magical ``Journey of the Magi.''

In each of these, and in most of the other panels, frescoes, and illuminated manuscripts in this exhibition, the viewer is confronted by an approach to art that is, in certain ways, remarkably ``modern'' in spirit and execution.

One might even go so far as to say that anyone delighting in the works of Klee, Morandi, or Magritte would be equally delighted by these Sienese paintings. Sassetta's and Klee's ultimate objectives may have been different, but the various ways each set about actualizing those objectives through color, line, and pigment were often quite similar. Stylistically and philosophically they were worlds apart, but as draftsmen, colorists, and designers of smallish, highly imaginative and provocative images, they often had a great deal in common.

Unfortunately, this beautiful and very important exhibition will not travel. It closes at the Metropolitan on March 19.