"I suppose I should justify this program a little," intoned droll conductor Christopher Hogwood to the standing-room-only National Symphony audience Thursday night at his all-Mozart program. "You see, it's meant to be a sort of antidote to 'Amadeus,' at least to the extent that what we are doing here is based on fact."

Well, it did need an explanation.

For example: there was nothing unusual about opening with the "Haffner" Symphony (No. 35, K. 385). Toscanini used to do it. Karajan does it. But what neither of them would do would be to open with only the first three of the four movements, as Hogwood did. The conductor didn't entirely eliminate the scampering finale; he just waited awhile to play it. He waited, in fact, until the end of the concert -- about an hour and half, and zillions of notes, later.

Even Glenn Gould never got quite that outrageous.

Hogwood, a superb musicologist in addition to being a marvelous musician and an ingratiating stage personality, came up with the ultimate answer. Mozart once did it that way. And he drew from the breast pocket of his tails a copy of a 3 1/2-hour marathon that Mozart once staged in Vienna with the "Haffner" arranged just this way -- like "bookends" on the rest of the program, observed Hogwood.

Obviously both the composer and the conductor have a perfect right to do this with the "Haffner." But each would have to admit that it makes for a certain lack of continuity. The third-movement minuet of this incomparably fluent, joyful symphony ends particularly inconclusively -- as if to be leaving the way open for an immediate pounce into the finale. When the prolongation, however, is more than three times as long as the symphony itself, a little bit of the zip is lost.

Hogwood is a specialist in authentic 18th-century performance practices -- gut strings, short bows, period flutes and all that. Technically there was no such thing in those days as a conductor in the modern sense. Mozart directed seated at the continuo instrument -- often a harpsichord. And that is what Hogwood does in his Mozart symphony recordings with the Academy of Ancient Music, the fine period ensemble he organized in England. His recordings with that group have sold so well that he has become a celebrity on the American symphony circuit.

He couldn't expect period instruments of the National Symphony, which he is conducting for the first time. But he did apply numerous authentic stylistic graces, and -- most of all -- there was the period program format.

The performance of the "Haffner" was scintillating. Even using players unfamiliar with 18th-century techniques performing on contemporary strings and bows, Hogwood managed to draw from the NSO a partial facsimile of the 18th-century sound -- transparent timbres, impeccable articulation, with dynamics that hewed close to the chamber scale.

The four works that came between the "Haffner" bookends were diverse. The high point was Horn Concerto No. 2, K. 417, with Barry Tuckwell. And NSO first flutist Toshiko Kohno's playing of the little C major Andante, K. 315, was beautifully colored and phrased.