Why won't famed conductor Valery Gergiev permanently relocate to one of the great cities of the world? Because he's already living in one of them.

In the cramped practIce audItorIum inside the venerable Mariinsky Theatre in St. Petersburg, the soprano rehearsing the lead role in the opera Tristan and Isolde is in nearly perfect voice. The orchestra sounds excellent, as well, at least to mere mortals. The company seems ready to move downstairs into the theater's magnificent gilded auditorium - which harks back to the glory days when Catherine the Great ruled this imperial city and the rest of the Russian Empire - to play before another packed house.

But Valery Gergiev, the dark-haired conductor on the small rostrum, is not content. He claps his hands overhead, signaling the orchestra to stop. In a soft voice, he suggests to the violinists that they sound like cows. Their sound is too noisy, too jumbled, and they must play each note with more precision, so that they sound separate and distinct. Making a slight clucking noise designed to mimic a violin, he indicates the rhythm he seeks.

"I want more sharpness, more quality," he says gently, with no trace of rancor or condescension, then gives the signal for them to start again. The orchestra seems to soar, then blends perfectly with the soprano's voice, thrilling the handful of visitors. But a few minutes later, the conductor halts them again and makes the same point in a different way, trying to nudge the players to a higher plane. He is patiently looking for a sound of transcendent beauty.

This is the Valery Gergiev the world rarely­ sees. He is most often observed on center stage in New York, London, and Paris, wearing black tails and receiving standing ovations, rather than wearing unpressed slacks and a casual black shirt in a rehearsal space far from the public eye. Gergiev - principal guest conductor of New York's Metropolitan Opera and set to become the principal conductor of the London Symphony Orchestra in 2007 - has been proclaimed "the greatest conductor alive" by critics at the Times of London and elsewhere.

At home, however, where he is general director of the Mariinsky Theatre and its famed Kirov Ballet and opera companies, Gergiev is a teacher - for the most part. He's also the man largely responsible for keeping St. Petersburg's glorious reputation as a world center for the arts flourishing after a difficult and dangerous transition from the Communist era.