By the time composer John Harbison gets to the most famous (and last) lines of F. Scott Fitzgerald's "The Great Gatsby," it's about 11:15 in the evening. Of course, no opera based on one of the best-known of all American novels can avoid this stunning summation--"And so we beat on, boats against the current, borne back ceaselessly into the past"--but the words are so familiar, so expected, so necessary to the novel's impact that their arrival feels anticlimactic.

Harbison's new opera, which premiered at the Metropolitan Opera on Monday night, is one of the biggest events of the season. For the Met to commission an opera based on such an iconic text is like announcing that the American opera tradition is about to receive its defining work.

"The Great Gatsby" may not be that, but it is an ambitious opera filled with tableaux that any American who didn't sleep through eighth grade will recognize as archetypes of our restless and sexually greedy society.

The press about the new opera has focused mostly on the composer's treatment of the Jazz Age musical vernacular: fox trots, ragtime, the Lindy. The American opera house has long been a melting pot for popular forms, especially recently. Harbison's opera follows William Bolcom's "A View From the Bridge," a work premiered in Chicago in October that is also heavily influenced by American popular music forms. These two works are enough to set American critics obsessing (once again) about the problems of mixing high and low music.

Harbison's solution--to compose his own popular tunes, with lyrics by National Public Radio cultural programming director Murray Horwitz--is more successful than most. The composer's sultry tunes are recognizable but unplaceable, and do not distract from the composer's own, often directionless chromatic modernist idiom. Horwitz (who receives surprisingly high billing on the program) writes suitably superficial and mindlessly amoral lyrics. Harbison's pastiche songs flow from the radio and the dance bands onstage during Gatsby's brilliant parties; they are both wallpaper for the dramatic action and fundamental to the musical score.

But there's been too much focus on this, as if Fitzgerald's novel were merely a little bit of plot grafted onto lots of musical references, a morality tale waiting for a composer to jazz it up. Fitzgerald's novel is stronger than that, and while Harbison's self-crafted libretto struggles to keep intact the beauty of the book's language, the immediacy of the jazz elements overwhelms the torrents of poetry in the original. The deepest challenge of setting this story is not dealing with its frequent, offhand musical references; it's dealing with its stark and swift drama.

Ironically, the new opera compresses the novel into 10 scenes yet lasts more than three hours. Harbison's libretto covers the essentials--the drinking and carousing, the infidelity, the accident, the denouement--but eliminates foreshadowing (sometimes at the expense of character, especially Gatsby's).

The party scenes, composed in a nonstop, "processed" jazz idiom (snippets of syncopated themes are woven together with almost Wagnerian obsessiveness), allow the composer to cover acres of narrative territory in the work's most accessible style: vocal lines that are crafted to fit neatly into the sliced and diced fox trots flowing from the orchestra. Thus Harbison sets text without resorting to old-fashioned vocal lines (as Bolcom did in "A View") or the aimless wandering between ungainly intervals that characterizes so much 20th-century vocal writing.

Unfortunately, despite the deft condensation and energetic music, the opera crawls at a snail's pace compared with the book. Worse, it never coalesces into a defining moment. The famous green light that stands for Gatsby's consuming desire to create his own perfect world with Daisy as the piece de resistance is a recurrent image (beautifully done by lighting designer Duane Schuler and set designer Michael Yeargan) but not an organizing one. Gatsby's death and funeral are the closest the score comes to catharsis but are too little too late in a score that generally lacks emotional density. Even the car accident, which at first impeaches and then confirms Gatsby's character, doesn't yield anything emotionally elemental.

An aura of respect pervades the evening. Harbison has respected America's own Greek tragedy just a little too much, creating something admirable but often dull. The Met has respected the work by hiring a very fine cast, with one unfortunate exception. For the title character, they have chosen tenor Jerry Hadley, who on Monday did not have the vocal flexibility or top notes to capture Gatsby's sentimental underbelly and heroic decency. An inadequate title character is a severe handicap. One leaves the novel still a bit in awe of Gatsby; one leaves the opera thinking the usual things about tenors.

The rest of the cast, however, is starry. Lorraine Hunt Lieberson, a prodigious mezzo-soprano known particularly for her brutally direct baroque performances, makes her Met debut as the sassy, but not so classy, Myrtle Wilson. She sings the role like a lounge singer, and it's one of the most effective performances of the night. Dawn Upshaw is a mesmerizing Daisy, at ease with the flapper mannerisms (her hands are in constant motion) and comfortable with Harbison's significant vocal demands.

For some reason, Harbison unexpectedly makes Gatsby a tenor and the narrator, Nick Carraway, a baritone. Dwayne Croft takes all the callowness out of his Nick, singing him as very much the equal of the decadents who surround him. His diction, compared with Upshaw's, was disappointingly thick. Susan Graham's Jordan Baker, the lean, brawny and amoral girl on Nick's arm, flounces through the opera in fine voice and with a dramatically suitable detachment.

The Metropolitan Opera Orchestra, which should be growing tired of all the repetitive compliments, deserves yet more. Harbison's score demands both stark dissonances and all the slithering, rubbery openness of a period dance band. Under the direction of James Levine, it delivers. So, too, Yeargan's stage design, which works miracles with a few pieces of furniture and some diaphanous scrims.

The production runs through Jan. 15.

CAPTION: Dawn Upshaw and Jerry Hadley in John Harbison's "Great Gatsby."

CAPTION: Jerry Hadley, left, and Dwayne Croft in the Metropolitan Opera's "The Great Gatsby."