A few members of the audience (presumably those who knew a bit of Italian) chuckled softly during some of the most serious moments of "La Sonnambula," Saturday night in the Terrace Theater. For a modern audience, this is an opera that sometimes trembles on the brink of unintentional comedy -- only to be redeemed again and again by the limpid beauty of its melodies. There is only one valid reason for presenting this simple-minded melodrama with its easy-listening tunes; you must have a group of outstanding singers to put on the stage.

The Washington Opera fulfills that requirement splendidly in the production it will be presenting throughout December. And in the Terrace, it has an ideal environment for bel canto opera: a small hall that puts no strain on fragile voices; acoustics that present the tiniest refinements of vocal technique with total clarity; superb sight lines and an intimate environment that gets the audience involved. Performing "La Sonnambula" involves considerable risk. Opera fans even in a production without surtitles must make allowances for its absurdities. Ideally they should love it for its absurdities as ballet fans do for "Giselle" -- and if the singing is not close to perfection the whole project loses its raison d'e tre. On opening night, the gamble paid off lavishly.

Melody and absurdity struggled for dominance most intensely in the first sleepwalking scene. Melody triumphed because it was entrusted, at this point, to three excellent performers.

Amina (soprano Janice Hall) had just walked -- silently, tentatively -- into the bedroom of Rodolfo (bass-baritone Jeffrey Wells), who was spending the night in the inn of her Alpine village. The scene was observed from a hiding place by the innkeeper Lisa (soprano Cyndia Sieden), who had been flirting with Rodolfo before the interruption.

Amina, dressed in a long, white nightgown, was obviously fast asleep, though her eyes were open. Her arms were stretched out before her, and in one hand she held a lighted candle. As she groped her apparently aimless way toward Rodolfo, she sang with an exquisite mixture of joy, pathos and vocal acrobatics about her love for Elvino (tenor John Aler), whom she would marry the next morning. Was he still jealous about the handsome stranger (this same Rodolfo) who had complimented her so lavishly?

"Come close to me," she sang, her voice running through incredible arabesques. "You know I love only you." Unfortunately, as she sang this touching testimony of conjugal fidelity, she was approaching Rodolfo -- and being watched by Lisa, who knew nothing about sleepwalking. Rodolfo behaved like a perfect gentleman -- blew out her candle and put her to rest on his bed while he went off to get help. Meanwhile, Lisa (who had once hoped to marry Elvino herself) ran to spread the gossip, and a chorus of villagers came in to pay their respects to Rodolfo, who had been recognized as the new lord of the local manor.

That's quite enough of the plot; a fairly standard theme of lovers' misunderstanding is given a new (if charmingly absurd) dimension with the sleepwalking gimmick. This leads to a smashing finale in Act 2, after the bridegroom rushes in to find his bride lying in another man's bed while the whole village looks on.

The sleepwalking also gives a new twist to another convention almost as old as opera itself: the mad scene. Composers had long fostered the illusion that insanity is good for the voice -- specifically, that it makes people sing lots of high notes and tricky coloratura. Vincenzo Bellini extended this convention to include sleepwalking in two climactic scenes -- one to get Amina into trouble and the other to get her out of it. They are not quite as spectacular as the mad scene that Donizetti would write for "Lucia" or as dramatic as the sleepwalking scene that Verdi would write for "Macbeth," but both of these high points in 19th-century opera probably owe something to Bellini's example.

Bellini's two scenes are musically appealing and theatrically effective when they are well performed, and Janice Hall does them superbly. Her voice is well-controlled, tonally rich and impressively agile from the mezzo range up into the highest coloratura reaches. She may have sung three notes that were not perfectly formed on opening night, but surely no more.

As an actress, she is always effective if seldom flamboyant -- a quality that would be alien to this role, in any case. Flamboyance was in the competent hands of Sieden, who brought the more vivid and mildly villainous role of Lisa spectacularly to life, with a range of facial expressions that were worthy of Carol Burnett and marvelously effective in the Terrace's close-up environment. Vocally, she has some splendid top notes and an impressive ease in ornamentating a melodic line. Her tonal production was not quite as even as Hall's, but still highly impressive.

The male roles in bel canto opera are usually harder to fill than the women's, and the Washington Opera has succeeded splendidly for this production. John Aler, a specialist in the bel canto tenor repertoire, has exactly the right kind of light, pure tone, the necessary high notes and the speed and grace of vocal movement that this music requires. There are not more than a handful of tenors in the world who can sing this kind of music so well, and his presence is one excellent reason for having a "Sonnambula" in town. Wells is impeccable, as usual: smooth and graceful in his singing, commanding and convincing in his stage presence. Stephen Dupont and Judith Christin both perform well in supporting roles and David Rees makes much out of little in the walk-on role of the notary.

The chorus was clearly having fun in this production, not only in the ear-caressing melodies but in many small details of stage business -- for which director Peter Anastos presumably deserves some credit. He is engaged in a transition from ballet choreography to opera directing, and this production augurs well for his future in his new field. Conductor Joseph DeRugeriis is a generalist who has conducted everything from Mozart to "Mahagonny," but in this production he sounds like a bel canto specialist and on opening night he had the small orchestra playing well.

Zack Brown's scenery is, as usual, enchanting -- this time imbued with a light-hearted bucolic flavor. On opening night, however, some of the men's costumes looked as though they might profit from one more fitting session.