Pianist Alicia de Larrocha's recent recording of Mozart's Concerto No. 19--civilized, respectful, lightweight, safe, eggshell-thin, undistinguished--left one unprepared for her performance of the same work with Leonard Slatkin and the National Symphony Orchestra at Wolf Trap Friday evening: taut, colorfully lyrical, fizzy.

Nothing adequately explains such transformations, but this concerto rises or falls on the opera buffa interplay among piano, strings and woodwinds (the latter appearing sometimes as a diverse chorus, sometimes as sassy individual singers), and Slatkin had the orchestra beautifully keeled and acutely balanced--this was far more than mere accompaniment. Using the orchestra as an easement, de Larrocha transported us into Mozart's unique operatic universe. When in the third movement the orchestra's accumulations surrounded the pianist with light-as-a-feather counterpoint, and de Larrocha responded joyously in kind, the effect was magically seductive.

After intermission came Berlioz's "Symphonie Fantastique," a nightmare of obsession fed by the composer's opium dreams. Although the composition is tight (Berlioz took much of his scaffolding from Beethoven's symphonic dramas), the dazzling orchestration often invites tidal-wave crescendos, ominous moans, grunting winds, tawdry evocations of a nostalgia that never was, a waltz backlighted with vulgar effects, menacing punctuations. Instead, Slatkin delivered analytical clarity, precise execution, graceful phrasing and always a sense of forward motion. But when the score virtually demanded powerful insistence, the NSO supplied it. In sum, this was a notably good, objective performance short on diablerie but replete with beguiling niceties (the cornet obbligato in the second movement took pride of place, but all the solos were excellent).

The clear orchestral detailing in the program opener--Rossini's "William Tell" Overture--made this war horse sound like a thoroughbred.