Before the opera "Marilyn" received its world premiere on Wednesday night by the New York City Opera, it seemed headed for a renown only slightly less grand than its movie-star subject's. With music by Ezra Laderman and a libretto by the poet and playwright Norman Rosten, this portrait of Marilyn Monroe's final months has been one of the most publicized new operas in years. All three performances (the others are tomorrow and Tuesday) were sold out in advance.

The work, a City Opera commission, opened a world-premiere festival in honor of the company's 50th anniversary, in which three new works are being presented on consecutive evenings. But unless one comes to "Marilyn" already obsessed by Monroe's mixture of come-hither glances and childlike behavior; unless one finds her death tragic rather than merely sad, there will be very little in this opera to justify three hours of dutiful attention. This is primarily an opera for the already converted. And even then, I wonder.

It is composed of vignettes from Monroe's life in 1962, presented in the form of flashbacks, memories and imagined scenes. Aside from Monroe herself (who is given an impressive physical impersonation if not a sultry vocal one by the soprano Kathryn Gamberoni), there is a Psychiatrist who keeps trying to get Marilyn to behave; a Senator, resembling Robert F. Kennedy, who has a fling with her before flinging her aside; two Moguls who treat her like a studio prop, and an ex-husband, Rick, who is meant to be a composite of Marilyn's three spouses (James Dougherty, Joe DiMaggio and Arthur Miller). This generic conceit replaced personality with mere role playing.

Musically, the opera's effect was often of an amiable lugubriousness. The score is never less than professional, and is often skillfully eclectic. But Mr. Laderman, who is the dean of Yale University's School of Music, tended to create genre music using the most obvious associations: expressionistic atonality for Monroe's unhappiness, episodes of boogie-woogie and jazz to represent her wild character, elements of popular dance rhythms (and a motif from "Hooray for Hollywood") for the Moguls. A central Trio has some striking effects, but the close-grained musical detail that might delineate character is missing.

The libretto doesn't help matters. Aside from one scene of confrontation with the Senator, there are no central dramatic conflicts, but only a succession of iconic scenes. Mr. Rosten, who was a close friend of Monroe's in her last years, is still clearly struck with her, so much so that he resorts to every known cliche about her as victim, sex goddess and innocent. "Each man dreams his own Marilyn," says an unnamed Man. Sometimes the portentousness becomes mystifying: "You're too beautiful to run out of anything," the Senator tells Monroe, "except your life." A conversation between Marilyn and her dead mother (in which the film star speaks her mother's words as well as her own), recalls not Norma Jean but Norman Bates.

There were strengths in the midst of the miasma. The production, conceived and designed by Jerome Sirlin, was generally simple and often masterly. The sets were projections on scrims and screens, creating, for example, beautiful starlit nights over palm trees, images of Hollywood night spots, a city skyline, a giant red-tinted American flag. Ms. Gamberoni's Marilyn was supported by a strong cast that included Ron Baker as the Psychiatrist, Philip Cokorinos as Rick, and Michael Rees Davis as the Senator, along with able conducting by Hal France. Discussions of acting are often irrelevant to discussions of voice, but Ms. Gamberoni's lyric soprano had more impact because of the able impersonation behind it. She even captured some of Monroe's upper-register quaver. One problem was that vocally she lacked both the lower-register support and the sense of sexually charged allusion that Monroe commanded.

A more serious problem may have been in conceiving of this as an opera in the first place. To my perceptions, Marilyn Monroe was not really a towering diva figure. She achieved mythic status only in the fantasies of her fans. She was actually a composite of artifice and sincerity, pre-pubescent charm and sexual manipulation; this makes her seem too ordinary for the opera stage. She required the screen and the still photo to magnify her; she would probably seem dwarfed by the artistic apparatus of any opera. In this opera, we can never even develop any sympathy or fascination: all we see is her self-indulgence and coy posing, which are far more annoying than the worshipful creators of this work know.

This opera is also part of a trend in recent American operas that treat central characters with such reverence that dramatic considerations are less important than recounting representative events for devotees, as if the stories had the status of lives of the saints. That was one of the weaknesses of Philip Glass's otherwise compelling "Satyagraha" (about Mahatma Gandhi). That was the case with Anthony Davis's "X: The Life and Times of Malcolm X." Even the painter Frida Kahlo has received the treatment.

Now Marilyn has joined the roster. At the end of the opera, wearing a teddy and wrapped in a red satin sheet, she stumbles up a flight of steps and dies of an overdose of pills, sprawled on the altar of a church. Coming after two generations of Marilyn hagiography, Marilyn conspiracy literature and beard-pulling broodings about Marilyn in American culture, this opera's unintended contribution to the Marilyn industry is actually to make it all seem extraordinarily silly. Marilyn Opera in two acts by Ezra Laderman; libretto by Norman Rosten; conductor, Hal France; production designed and staged by Jerome Sirlin; co-director, Paul L. King; costumes by V. Jane Suttell; lighting by Jeff Davis. At the New York State Theater. Marilyn . . . Kathryn Gamberoni Rose . . . Susanne Marsee Vinnie . . . Michele McBride Senator . . . Michael Rees Davis Psychiatrist . . . Ron Baker Rick . . . Philip Cokorinos Moguls . . . John Lankston, Jonathan Green

Photo: Kathryn Gamberoni in "Marilyn" at the New York City Opera. (Martha Swope Associates/Carol Rosegg)