It would be hard to imagine a more effective or refreshing tonic for the soul, in these parlous times of ours, than the kind of euphoric revels the Royal Danish Ballet set forth for our delectation at the Kennedy Center Opera House last night, as the company launched a two-week stay in Washington.

Ever since its first appearances in this country almost three decades ago, the RDB has had a reputation for a special brand of bright-eyed, bristling zest--a combination of joie de vivre and joie de danser. This trait seems to have undergone renewal and reinforcement since the troupe's last appearance at the center in 1976 (the company performed in the U.S. in 1980, but only in Chicago). In the meantime, a new director, Henning Kronstam, has taken hold and a new generation of younger dancers has emerged, and the combination appears to be bearing healthy fruit. Pervading the atmosphere during last night's program was a sense of joy, of vigor, of warmth and conviviality; the dancers' eagerness to share the pleasures of their art seemed a tangible entity.

Some of the evening's halcyon air can be attributed to the present uplift of the company, some to an elusive quality that can be thought of as the Danish temperament, and the rest to the magisterial choreographic embodiment of that temperament in the work of August Bournonville, the 19th-century ballet master who reigned over the troupe for 50 years and put his bracing mark on it for all time. For the opening of its month-long tour, the company chose a pair of Bournonville treats--the complete "Kermesse in Bruges," in its U. S. premiere, and the scintillating Act III from "Napoli." The "Kermesse" is a masterpiece of comedic theater in all its dimensions; the "Napoli" extract is pure dancing in its most ebullient mode--side by side they make for uninterrupted delight.

The opening scene of "Kermesse" set the tone for the program. After a brief orchestral fanfare, the curtain opened on a stage that looked buttered in sunlight. Lars Juhl's exquisite set pictures the town marketplace of 17th-century Bruges, flanked by comfy Flemish houses in brick with their quaint, towered facades. Broad-sailed ships sit at harbor in the background. The peasant populace throngs the square, bedecked in warm pastels--mustard, rose, olive, yolk yellow. A pair of nobles in feathered finery arrive with a flourish. A high-flying jester scoots from side to side with tricks. A couple is borne aloft, sitting atop a wine barrel. Wooden clogs and toe shoes mingle on the floor. The Burgomaster enters in a processional to preside over the Kermesse--the town's annual church fair. All at once, the stage is alive with dancing, and three couples in rich, dark green move forward to present a Slovanka, arms folded, heels snapping together, bending in deep plies, lining up to blow kisses to the audience. Only then does the exposition of the actual story commence.

The plot, typical for Bournonville in its mixture of earthiness, fantasy and jovial morality, is at once absurdly complex and simple to follow in the clarity of the Danes' theatrics. In essence the tale is this: in reward for the rescue of his daughter Eleonora from the clutches of lecherous aristocrats, the alchemist Mirevelt gives three young men each a magic gift--a sword that insures victory, a ring that makes everyone fall in love with the wearer, and to Carelis, Eleonora's suitor, a viol that causes listeners to break into ceaseless dancing. A series of adventures ensues--the love-ring, in particular, provokes a droll imbroglio--and in the end the three swains, including Carelis, are happily reunited with their sweethearts. The final crisis is hilarious--Mirevelt and two of the young men are about to be burned at the stake for sorcery, when Carelis' viol sets everyone--monks, soldiers, onlookers and the condemned--to wild jiggling in time to its rhythmic merriment. Enchanting Lis Jeppesen and brilliant, young Bjarne Hecht were last night's Eleonora and Carelis; the large cast included not only children but such splendid RDB veterans as Niels Bjorn Larsen, Fredbjorn Bjornsson, Kirsten Simone and Niels Kehlet, demonstrating where the Danish cachet for vivid characterization and mime comes from. As in "Napoli," which introduced a further slew of young virtuosos, the dancing in "Kermesse" ranged from folk patterns to the filigreed effervescence of Bournonville's classicism. As an aperitif for the company's extended visit, the evening could scarcely have been improved upon. The program is being repeated, with changes in cast, tonight and twice on Sunday.