Dominick Argento's "Postcard from Morocco" casts a spell of unusual power for those willing to enjoy its decidedly different concepts. Last night, in an admirable performance, it was the second opera in the Kennedy Center's Terrace Theater Summer Opera, where it looked marvellous, giving the intimate surroundings a special flavor.

At every moment "Postcard" is an eminently vocal score with brilliantly colored support from an eight-player orchestra of violin, viola, double bass, clarinets and sax, trombone, guitar, percussion and piano. The variety of rich textures Argento draws from the combinations possible in his pit is one of the most rewarding aspects of the entire score.

His imaginative libretto by John Donahue has elements of fantasy mixed with memories of an era around the turn of the century to whose nostalgic moods Argento provides several keys. His score, both in fluent vocal writing and in some exquisite instrumental passages, has touches of Richard Strauss alongside irresistible waltzes that could be called "Fascination." "Seduction" or "Tantalize Me!" Tall potted palms onstage as well as in the midst of the orchestra bring days and nights in sultry palm courts quickly to mind.

The seven characters spend the opera in the waiting room of "a railway terminal in a Middle Eastern country." There is a lady with a hat box, one with a cake box and one with a hand mirror; one man has old luggage, one a paint box, one a cornet case and one a shoe sample kit. No one will open his box. Each, in solo scenes, suggests a vague past, while various voices are combined in elegant duets, trios, quartets and so on up to septets, both sung and at times hummed, that greatly enhance the score. Two mimes add further theater in straight scenes and in a dance duo.

And there is a passage of amazing effect when the orchestra, its platform raised, launches into "Souvenirs of Bayreuth," in the manner of the best hotel or railway bands of nearly a century ago.

They play what used to be called "strains" of "Lohengrin," in the Bridal Chorus and Third Act Introduction; "Tannhauser," in the "Evening Star," and motives from both "The Flying Dutchman" and even Siegfried's Funeral March. In this scene, Frank Toperzer's percussion won an outburst of applause, to which David Perry's lovely guitar, James Badolato's winds, and John Beeson's piano were equally entitled.

The Argento score, more than most operas today, demands not only special singing actors, but long passages of singing of an "interior" kind: not simply soft, but introspective in quality and texture.

On the whole the singing was very good with some outstanding contributions. First among these, both for her versatility in an instantaneous switch from a pigeon-toed eccentric old woman to an ostrich-feathered vamp and for her magical projection of an aura of greatness in song and movement is Elaine Bonazzi as the lady with the hat box.

For straight vocalism of a high order William Dansby's cornet man was excellent, as was Wayne Turnage as the shoe man.Claudette Peterson tossed off phrases in the stratosphere with dazzling ease and Barbara Hocher made much of her grande dame with the cake box in which, she insisted, she keeps her beloved . . . "he is here at this moment . . ."

Tenors Dennis Bailey and Michael Best found some of their roles a trial, but they added strongly to the ensemble.

Any score as complicated as "Morocco" asks much of the conductor in terms of utmost flexibility in tempos, extremes of dynamic ranges and the sense of making chamber music. John Mauceri held everything together in ideal fashion, though he has a problem in the intonation of his violins.

"Postcard" is a remarkable work with few flaws in its 90-minutes course. Mr. Owen's final scene - he is the paintbox man - tends to drag. It would be better staged in the area where the mimes play rather than taking the time to set and reset the stage for what then unbalances things. This is an easy thing to adjust. But this is an opera not to be missed.