"Ridi, Pagliaccio" ("laugh, clown, laugh"): two words sung to five of the most intense notes ever written for the tenor voice. They are the climactic moment of Ruggiero Leoncavallo's opera "I Pagliacci" ("The Clowns"), which will be shown tonight on PBS (at 9 on Channel 26 and Maryland Public TV; stereo simulcast on WETA-FM).

As he sings, Placido Domingo smears white greasepaint on his face. He is the clown Canio, preparing to go on stage in a commedia dell' arte -- an ancient Italian folk comedy about a husband, wife and secret lover. Canio has a similar situation in real life, and doesn't find it funny. "Laugh, clown," he sings to himself, "and everyone will applaud . . . Laugh about the pain that is poisoning your heart."

Before the night is ended, his wife Nedda (Teresa Stratas) and her lover Silvio (Alberto Rinaldi) lie dead at his feet. "I Pagliacci" is a story of desperate sex, smoldering jealousy and sudden violence, comedy twisted into tragedy. In a good performance, it seizes audiences and transfixes them. Tonight's performance, directed by Franco Zeffirelli with Georges Pre tre conducting the La Scala Orchestra and Chorus, is a good one.

Zeffirelli, aided by excellent acting from all his singers, underlines two points: the miror-relation between art and reality and the irony that the same situation (in this case, adultery) can be comic or tragic depending on its context, who is involved and how it is handled. Nedda's adultery is tragic because Canio is insecure: a clown by profession who wants to avoid all traces of clownishness in real life.

Domingo performs superbly. Vocally, he is in excellent form, and he immerses himself totally in the character. Stratas is an ideal Nedda. Her singing is better than it was for the more demanding music of "Traviata." She is earthy, passionate, wistful and a consummate comedian.

With less effective principals in the cast, Juan Pons could easily have stolen this show in the secondary role of Tonio.