"This performance has no intermission." Outside the Kennedy Center Concert Hall there is localized panic. How many cups of coffee at dinner? Don't think of water, trickling water, rushing water. Niagara.

A man consoles his wife: "They can't play forever. They'd just wear out." His prediction will come true.

Last night was Mahler night with the National Symphony Orchestra. Since Leonard Slatkin became music director-designate in 1994, he has performed a Mahler symphony each season--a wise idea, given the need to refashion the orchestra. Mahler makes musicians walk the line.

Last night it was Symphony No. 7, the least performed and least known of the composer's symphonies. It may not be his longest, but at 80 minutes it's a major commitment for the listener; its loose-limbed structure, weird cacophony of reconfigured nature sounds and Felliniesque humor make it seem even larger and more sprawling than it is.

Mahler's Seventh confronts the listener like a big Bruegel painting, teeming with . . . teeming with . . . the kind of stuff that teems. It's hard to be more specific. It's like a huge family reunion of Mahler's usual cast of characters, with the exception of the Hero, who hasn't been invited. Replacing him is a fiendishly sly street musician with appalling manners. There's a lot of bustle and posing and people who may be laughing with or at each other. Malice and drink.

The Seventh has suffered from its own brilliance. The Second Viennese School--Schoenberg, Berg and Webern--loved its dazzling and daunting wealth of innovation and bizarrie (it was their favorite Mahler). In their own time, alas, these composers were inverse prophets: They consistently bet on the wrong horse for all the right reasons. More typical was the critical reaction, especially in this country, in which the words "pruning shears" recur like a bad penny.

The Seventh has always suffered from unfortunate comparisons with its siblings, the homely middle child who has no merits of his own but is merely a reflection of his brothers' virtues. Thematic ideas from other Mahler symphonies dance in and dance out throughout the work's five movements; it sounds like Mahler, but it's not really wallowing music. It's jittery and high-strung and it can't play football. Mahler, perhaps perversely, thought it was some of the happiest noise he ever composed, though this sounds a bit like an obscurantist painter pointing to his latest Rorschach blot and saying, "It's all about joy, man."

The Seventh's real substance and depth, which like an e.e. cummings poem is on the level of grammar and syntax, eluded most early listeners. With 10 Mahler symphonies to choose from, why perform the one that has all those disconcerting tics and other bad habits?

Slatkin and the orchestra demonstrated exactly why, in a performance that revealed solid technical preparation and emotional commitment--and suggested ample room for growth and greater confidence in future (one hopes) performances. Slatkin knows his Mahler very well, and while he was not able to get the orchestra to play with the Vienna-in-its-veins assurance of a top Mahler orchestra, he got the essentials across: the rhetoric, the atmosphere, the pacing and the quirkiness. The foundation was there, and it was solid.

The first movement (this is happy music?) elicited a small grunt from the conductor, more of a lifting-the-groceries grunt than a Glenn Gould grunt, but testament nonetheless to Slatkin's powerful engagement with the music's barely restrained chaos. In the third movement--the proverbial "grotesque scherzo"--every rude noise was carefully placed, the disruptions so well timed that one could practically smell the indifference to propriety. In the two "Nachtmusik" movements--2 and 4, which were the original core of the symphony--the control was even and steady, the endings finely wrought, the twitchy but hushed instrumental dialogue maintained with careful attention to dynamics. And the last movement was a study in suspended judgment: Is it banal, or ironic?

Somewhere in the middle of the first "Nachtmusik" came the first thoughts of water. Nice thoughts.

Imagine returning from vacation to the big city and listening to the way the night sounds, wishing perhaps that it sounded more like a seacoast or waterfall than the soundtrack to "Cops." The imagination kicks in and suddenly, if you listen right, the urban hubbub changes, becomes something soothingly consistent, despite its surface eruptions of random noise. It begins to sound like rushing water.

Mahler reimagines the ambient noise of the world in the Seventh. Like an impressionist, he comes closer to hearing the world in all of its unprocessed beauty. It's disconcerting, it's wonderful, and it's honest. By the end of last night's performance, the horns were getting tired, and there a few more squawks than usual from the wind section. Perhaps the musicians were worn out, but it was honest fatigue. They had reconfigured the world into something both true to itself, and more beautiful than itself, and that's not easy.