Even the 68-year-old Gian-Carlo Menotti may have lost track of how many operas have sprung from his busy pen over the last four decades. Some were memorable and some, particularly in later years, weren't.

But in the late '40s and early '50s he was composing work after work that stands the test of time. "Amahl and the Night Visitors" is the best known of all contemporary operas, because of its quality and its broad exposure as an NBC Christmas classis. NBC commissioned the show. Tonight Channel 26 brings us an even stronger opera, "The Medium," written for the stage in 1946, five years earlier than "Amahl."

The central character is a spiritualist named Madame Flora (or, in her more bizarre moments, Baba) who specializes in cheating the bereaved by making them think they are reestablishing contact with the dead. Eventually the guilt feelings this engenders drive her into madness, violence and homicide. Lest one suspect that this is merely an esoteric work about a troubled person, irrelevant to most our lives, be assured that Menotti broadens the issue in his text to the delicate grip most persons have on reality. Also, the music is often lovely, especially the cradle song, "Oh, Black Swan," which appears in both acts, and the ensembles of the parents of the dead who come in desperation to Madame Flora.

One does not recall a singer of greater stature than Maureen Forrester taking on a Menotti role, as she did at the 1974 Stratford Festival, from which this telecast derives.

Better known throughout her long career as a concert artist than as an opera singer, Forrester makes one wonder why. The gradual sequence of emotional stages by which she slips from disturbance into madness are agonizingly delineated.

Equally fine is the performance of Stelio Calagias as Toby, the mute street gypsy she has adopted. His may well be the only nonsinging central role in an important opera.

Shawna Farrell is Baba's daughter. Gino Quilico, Rosemarie Landry and Barbara Ianni are the parents. The production is from the Canadian Broadcasting Company. There will be a stereo simulcast of the 10630 p.m. telecast on WETA-FM.