Abstract painting. What a concept. Color, line, shape. Maybe some brushwork, maybe not. That's about it. You can do what you want with the results. You can treat a picture as a window onto something (open sky, inner space). Or as a physical object with texture and weight. Or, if you know a little history, as a utopian gesture, a chunk of the social sublime.

Abstraction has been around so long now, though, that it's hard to remember how radical it felt when it was new, what it demanded of an audience in terms of faith and flexibility and, if you got it, how stimulating it could be. Some sense of that excitement comes across at the Americas Society this fall, in a trim, tangy exhibition titled ''Abstract Art From the Río de la Plata: Buenos Aires and Montevideo, 1933-1953.''

Buenos Aires and Montevideo, the capitals of Argentina and Uruguay, are in the south of South America, a long way from the hot spots of early Modernism in Europe and the United States. Yet in the 1930's and 40's, these two cities, on opposite banks of the Río de la Plata, were lively places for a forward-looking artist to be.

Both were cosmopolitan centers with large immigrant populations from Italy, Germany, Spain and Eastern Europe. The years before and during World War II brought a further wave of Continental arrivals, among them artists either coming for the first time or returning after a time away.

One of those returning was Joaquín Torres-García (1874-1949), a native Uruguayan who settled in Montevideo in 1934 after an impressive career abroad. A painter, a sculptor and a teacher, as well as a cultural anthropologist and a maker of toys, he knew Picasso in Paris and Duchamp in New York. Personally, he took up a lot of space, as visionaries often do -- you either gave yourself over to him or ran in the other direction -- and he had big plans for art in his homeland.

He wanted to create a South American version of Modernism, a progressive, socially engaged, Arts and Crafts aesthetic that would rival new art in Europe but be different from it. Forget those icy, uptight northern styles, he urged his disciples. Invent a warm, humane, superior ''art of the south.''

Yes, O.K., but what, exactly, might that be? One idea was that it would mix European innovations with some form of indigenous art, the global with the local. But a ''local'' art was hard to find. Neither Argentina nor Uruguay was a site of major pre-Columbian cultures. And no nationalist style, like Mexico's brand of Socialist Realism, had been developed.

Torres-García finessed the problem by filling his own work with Constructivist grids and pictographic forms (a figure, a tree, the sun), a pictorial language that gave at least the illusion of being culturally specific and universal. Some younger artists followed his synthetic direction; but for others it was his purely abstract work -- three handsome examples of it are in the show -- and the uncompromising spirit it represented that were a model.

By the late 1930's, Buenos Aires and Montevideo were simmering with cultural energy and producing competitive aesthetic minimovements. Each had a distinctive name (Perceptismo, Concrete-Invention Art, Art Madí) and a theoretical platform, and each cooked up promotional manifestoes that claimed to offer the absolutely newest, truest version of the new.

Excerpts from these position papers appear in a catalog essay by the show's curators, Mario H. Gradowczyk, an art historian, and Nelly Perazzo, former director of the Museo Sívori in Buenos Aires. And the words are young people's words -- ''all nature is false, all expressionism is false, all romanticism is false,'' that sort of thing -- full of idealism and hustle, silliness and intellectual fizz. And if in the end, one movement tends to fade into another, some memorable styles and talented artists emerged.

The enigmatic Madí group, for example, along with the Concrete-Invention Art Association, produced a species of shaped canvases that combined Dada zaniness and Neo-Plasticist order. Carmelo Arden Quin's ''Méchanique'' (1947) is a neat geometric puzzle enclosed in a notched, irregularly angled frame, while Raúl Lozza's pointy, asymmetrical ''Composition'' (1945) could be a stained-glass window designed for a tilting cathedral. Madí sculpture could also be pretty wild: Gyula Kosice's ''Röyi'' (1944) looks like an oversize abstract toy, and in a sense it is: all the parts move.

Certain painters like Alfredo Hlito and Lidy Prati wore their influences -- Torres-García in his case, Mondrian in hers -- right on their sleeve before finding voices of their own. And Tomás Maldonado was basically an eclectic one-man art movement: his paintings in the show look as if they'd been done by several different artists, each equally gifted.

Among his best pieces is the large, spare ''Development of 14 Themes'' (1947). With its network of discontinuous dark lines and patches of pale color, it suggest an all but blank canvas that has been stitched and patched and is sagging a bit in the middle. The result is a stylish, wounded Minimalism nearly two decades before there was a Minimalism, and it made an impression on the coming avant-garde generation, particularly on Conceptualist pioneers like Lygia Clark and Hélio Oiticica in Brazil.

Mr. Maldonaldo gained an international reputation as a painter and teacher. Other artists had less success, in part because they were on the scene early, when abstraction was drawing a hostile press. Juan Del Prete, an audacious experimenter, exhibited in Buenos Aires in 1933. But when the show was critically slammed, he lost his nerve and returned to the figure. Because he could afford only limited studio space, he kept throwing out old work. Much of his output is gone.

Another artist, Esteban Lisa, perhaps unwilling to risk similar rejection, kept his paintings almost entirely to himself. Although he was a popular teacher and a regular art-world presence, few people saw his work until after his death. The six pictures at the Americas Society are a delight: tiny oil-on-board paintings in muted earth tones and an organic-geometric style somewhat like that of Arthur Dove.

Of all the works in the show, in fact, his are the ones you want to take home, stay close to, be protective of. Their size has something to do with this. So does their aura of personal touch and psychic adventure. You can almost feel the artist bending over his studio table or leaning toward his easel as he focused everything on a drama of color, line and shape projected from the inside rather than copied from the outside, a new, absurdly boundless sort of reality, which is what abstract art is.

''Abstract Art From the Río de la Plata: Buenos Aires and Montevideo, 1933-1953'' remains at the Americas Society, 680 Park Avenue, at 68th Street, (212) 249-8950, through Dec. 9.

Photos: Progressive ideas far from the hot spots of early Modernism: Top left, ''Röyi'' (1944) by Gyula Kosice; above, ''Méchanique'' (1947) by Carmelo Arden Quin, from ''Abstract Art From the Río de la Plata: Buenos Aires and Montevideo, 1933-1953.'' (Photographs from Americas Society Gallery)