Happy Poland! Not a single sad tone or tragic gesture could be discerned in the Kennedy Center Concert Hall last night as Mazowsze, one of the world's best-known folk ensembles, surveyed the dances and songs of the land that lies between the Baltic Sea and the Carpathian Mountains and the rivers Oder and Bug. A light nostalgia permeated some songs from Cieszyn; otherwise, all was fun, pride and vigor. If the two dozen numbers were not a biased sample, Poland is a nation with no legacy of melancholy, mourning or anger.

There was variety on that stage full of cloudless sunshine. Some differences were subtle and one had to look and listen closely to distinguish dances like the krakowiak and the krakowiaczek, or appreciate a hybrid polka-mazurka. Other contrasts were bold, as among the families of dances based on hopping, skipping or stamping. There were genteel dances like the polonaise and such feats of virtuosity as partnered cartwheels-in-air. Formations changed kaleidoscopically, from simple chains and circles to intricate wheels and interlocking shuttlecocks.

Technically and stylistically, the company brought off its bright portrait of Poland. A fresher, more handsome group of performers would be hard to find. Never did the great amount of good cheer seem forced. A considerable amount of ballet training was undoubtedly needed to produce the dancers' finely stretched body lines and ease in the air. It did not, though, betray itself through academicisms. While not all the best dancers may be the choicest singers, the impression the group gives is of individuals with both talents.

Responsible for the smooth staging, the restrained splendor plus generous diversity of the costumes and, presumably, the opitimism of Mazowsze is Mira Ziminska-Sygietynska. Witold Zapala is principal choreographer. Most of the folk music has been tamed as if for an operetta. Yet it was conducted with flair by Andrzej Potapczuk, whose orchestra was perched on the box floor to the right of the stage.

With all the thanks one gives these individuals for a fun evening, the program does leave one with a reluctance to return. No doubt, this is due not only to the one-mindedness of Mazowsze in particular but also to the inherent dilemma of all theatricalized folk art. We expect artists to evolve; yet the more the folk artists develop, the less they can lay claim to being "folk." On its next visit Mazowsze is likely to be much the same, down to the predictable inclusion of an American song. Was it, though, in the spoken introduction to this visit's piece of Americana that the darker side of the Polish reality showed? "We are a brave people," the audience was told "but we now need your help."