Jaap Schroeder, that eminent and inveterate proselytizer for stylistic and instrumental authenticity in pre-Romantic music, led a program with the Smithsonian Chamber Orchestra Saturday night that was daring even by his own rigorous standards.

He was not doing his usual thing, taking Baroque music and freshening it with original instruments and performance practices. The daring of this program was his taking familiar music from what is the early mainstream of modern symphonic repertory and playing it as it might have been done at its beginnings--but has hardly been played since.

The two principal works were quite familiar: Mozart's Ninth Piano Concerto (the "Jeunehomme") and the ecstatic Schubert Fifth Symphony.

The merits of Schroeder's approach, from the orchestral point of view, are debatable in this music. But it should be stipulated, before getting to that issue, that the performance of the Mozart on the fortepiano Saturday night by Malcolm Bilson was enchanting by any standard. The fortepiano is that transitional instrument between the harpsichord and the modern piano. The instrument's lyric range is superior to the harpsichord's but the days of sustaining pedals were yet to come.

For Bilson, a specialist in this period, the limitations often are turned to advantage. Mozart's rippling articulation is easier, and you don't have to scale down the dynamics the way the Mozart player on the concert grand must do--sometimes with an unsettling sense of effort.

With an artist of Bilson's expressiveness, the rise and fall of phrases flow more easily, and there is nothing more important to performing Mozart than an easy flow.

This approach does, however, cost a certain price by modern standards. It was in this concerto--composed when he was just 21--that Mozart first unleashed that volcanic intensity that surges just beneath the surface of the mature concertos. And in so eloquent a work as the concerto's passionate C-minor slow movement, one missed the capacity of modern instruments to swell to an almost shattering volume and intensity at the climax of the main theme. That is not to say that the result Saturday night was bland; it is just to say that a Serkin, for instance, can get even more out of that particular phrase on a concert grand. Still, Serkin would be very hard pressed to produce such articulation in the finale.

Schroeder, who conducted standing but also played first violin, had the music beautifully in hand.

The Schubert Fifth fared less well, though it was still splendid. There were two problems. The most important was that the legato lyricism of much of the symphony is so long-breathed that early instruments are hard pressed to make it really sing. By Schubert's time, also, the variety of tonal textures in the orchestra were taking on a complexity that often was beyond the early instruments. Still, in the madcap final movement, the capacity for articulation of these instruments was impressive.

The concert, which was at the Smithsonian's Baird Auditorium, opened with a short symphony by the lesser Haydn, Michael. It was cheerful, uncomplicated and showed why its composer is not to be compared with his brother, Joseph Haydn.