SEATTLE (AP) _ Flagstad and Melchior. Nilsson and Vickers. And now, Eaglen and Heppner.

The Seattle Opera wrote a glorious new chapter in the performing history of Wagner's ``Tristan und Isolde'' on Saturday night when it premiered a production starring soprano Jane Eaglen and tenor Ben Heppner.

It may be no exaggeration to call it the finest performance the world has seen of this astonishingly difficult masterwork in the past quarter-century.

Wagner poured all of his passion, his frustration, his rage and his longing into the nearly four hours of music that accompanies his retelling of the love between the Irish princess and the Cornish knight, their betrayal of King Marke, and their remorse and deaths. There is almost no external action _ the opera takes place in the hearts and minds of the hero and heroine, who must give voice to their innermost thoughts against the backdrop of a feverish orchestral accompaniment.

Eaglen and Heppner, emerging superstars just now reaching their vocal maturity, were singing the title roles for the first time anywhere, and both surpassed expectations.

Not since the retirement of Birgit Nilsson has there been a singer with the effortless amplitude of sheer sound to do justice to Isolde. Eaglen hit every high note dead-on with thrilling power in such outbursts as Isolde's curse, but she also caressed the vocal line with delicacy in quieter moments. Never has her voice sounded more focused, her interpretation more committed. At the end of the evening, as she stood over the body of Tristan and prepared to join him in death, she sang a ``Liebestod'' of melting beauty and freshness _ and sounded as if she could have gone on for another four hours.

Heppner matched her, note for gleaming note. His uniquely sweet heroic tenor stood up to every test, from the ecstatic love duet of Act 2 to the delirious monologue in Act 3 as Tristan lies mortally wounded at his ancestral home on the coast of Brittany. His abandon as he sees Isolde's ship arrive and rips off his bandages in excitement was both harrowing and deeply moving. Only a slight cracking on one note in that last scene suggested any sense of strain in his performance.

How rare, and how gratifying, to hear this grueling role actually sung, not barked as most Tristans are forced to do. The last tenor equal to the task was Jon Vickers, who sang it only toward the end of his career starting in the 1970s. Before that, you'd have to go back to Lauritz Melchior, partner to Kirsten Flagstad and Helen Traubel in the 1930s and '40s.

To be sure, there's more to ``Tristan'' than the two lovers _ though not much more. Speight Jenkins, the Seattle Opera general director who had long dreamed of uniting Eaglen and Heppner, saw to it that the production did not let them down.

As Isolde's faithful maid Brangaene, mezzo Michelle DeYoung sang ardently, though the famous ``Habet Acht'' in Act 2 when she warns the lovers of danger didn't float with ideal steadiness. As Tristan's sidekick Kurwenal, baritone Greer Grimsley displayed a black voice of prodigious power. Peter Rose's warm bass made King Marke's lament over his betrayal sympathetic instead of tedious.

Conductor Armin Jordan drew remarkable playing from the orchestra, which sailed through the long evening with barely a misstep.

Stage director Francesca Zambello, assisted by scenery and costume designer Alison Chitty, created a production that mixed ingenuity with excess. A large chrome cube placed center stage provided the effective core for all three acts, first as Isolde's stateroom with the ship's deck looming above her, then as her chamber in Marke's castle, and finally as Tristan's refuge. But why the bare-chested sailors apparently stoking the boiler aboard a sailing ship? And why a snowfall for Act 2, when Wagner specifies a summer night?

Zambello's best contribution was her sensitive handling of the chief protagonists. Eaglen, whose large size undeniably is at odds with the role of a legendary beauty, was given a minimum amount of movement and never allowed to look silly.

Eaglen and Heppner will sing their roles at seven more performances this month and will be reunited in the fall of 1999 in a new production of ``Tristan'' at New York's Metropolitan Opera. After that, with luck, the English soprano and the Canadian tenor should be poised for a partnership that will carry well into the next century.