Of the major romantic concertos for piano and orchestra, the Brahms No. 2 is unique. It's the grandest in scale, with four movements--like a symphony--instead of the usual three. And no other such work is more physically demanding for either pianist or the orchestra. But for all this enormity, the Brahms B-flat is more human, warmer than most such concertos--a work that is not so much melodramatic as mellow.

It was the lyricism, rather than the concerto's sheer might, that pervaded the glowing performance of it at Wolf Trap Saturday night, with Eugene Istomin the pianist and Mstislav Rostropovich conducting the National Symphony.

Time and again Istomin avoided the hard edge that many virtuosos prefer. Obviously one can't take the edge off many of the work's treacherous octave runs, but Istomin took care to ease into some of the normally jolting chords that dot the work, particularly in the first movement.

For all the expressive depths that the concerto plumbs, it is a work that builds to a climactic movement that is a buoyant, sometimes playful expression of joie de vivre--rather than the usual romantic conclusions of despair or triumph over adversity. Istomin seemed to be keeping that in mind throughout the performance.

It was in the last movement and the glorious slow movement that precedes it that Saturday's performance rose from the level of the quite good to the truly eloquent. The expressive gist of the slow movement is the interaction between the Elysian calm of the main theme, stated by the solo cello, and its juxtaposition with the ambivalent unease with which the piano responds.

Much of the piano's part is a monologue in which the material is poured out in a manner sometimes resembling extemporaneous speech. Istomin played it like poetry, with sensitive attention to shifting of meter and subtlety of nuance and tone. First cellist John Martin's playing of the serene main theme was just as sensitive. It was appropriate that at the end of the concerto Istomin snapped up, leaped over the podium and embraced Martin before he got around to acknowledging the cheers of the audience.

The joyful finale was just wonderful. Istomin took it marginally slower than usual, which gave him and Rostropovich a chance to indulge in all kinds of rhythmic and harmonic pointing that a faster pace would have precluded.

This gentle approach to the concerto was not without a bit of a cost, particularly in the sense of propulsiveness and sharp contrasts of mood in the first movement. For instance, when the main horn theme returns suddenly at the start of the coda, covered in a veil of mystery, it was beautifully played, but the surprise element would have been stronger with a tighter contrast to what came immediately before.

The occasion brought the finest Brahms conducting this listener has heard from Rostropovich. Brahms has not always come easily to him. But this time it was very good. The concerto had a solid pulse, richness of tone, sweep and splendid discipline, both technically and structurally.

After the intermission, he conducted eight segments from Prokofiev's ballet, "Romeo and Juliet," which Deutsche Grammophon will record here this week with the orchestra. The performances were tensile and precise.

But for the first time, the limits of the new sound system at the Wolf Trap Meadow Center began to show. Prokofiev's orchestration is so rich that crucial details, especially in the percussion, were not heard--as they would have easily been at the Kennedy Center Concert Hall