Two marvelous American chamber orchestras -- one sporting period instruments, the other their conventional cousins -- converged on Stillwater's Trinity Lutheran Church on Thursday for the penultimate concert of the International Chamber Orchestra Festival, an exhilarating four-week marathon that will still be spoken of a decade hence.

San Francisco's Philharmonia Baroque Orchestra (PBO), last of the four bands to have braved our icy clime, was onstage for the first half of the concert, the St. Paul Chamber Orchestra for the second. Linking the halves were the superb tenor Thomas Cooley, a Minneapolis native, and the bobbing, bubbling figure of Nicholas McGegan, the PBO's music director since 1985 and an SPCO mainstay -- first as Baroque Series director, later as artistic partner -- for two decades.

The PBO, whose mandate extends well beyond the baroque period, draws on a sizable pool of musicians, not all of them Californians. The ensemble works about 15 weeks per year, traveling to venues throughout the Bay Area. It has commissioned new music for period instruments and has collaborated with the edgy Mark Morris Dance Group. Among its notable recordings are Handel oratorios with the late Lorraine Hunt Lieberson and a recent disc of Beethoven's Ninth.

Thursday's program, much of it borrowed from the opera, encompassed the turn from baroque (Rameau and Handel) to classical (Haydn and Beethoven). And if Haydn finally took the laurels, Jean-Philippe Rameau was the evening's revelation.

College textbooks of 40 years ago lauded the long-lived Rameau as the great French musician of his era. Felicitous performances were nonexistent, however, and skeptical undergraduates were unconvinced. But lo, the textbooks were right. Rameau's complex, kaleidoscopic orchestral textures come alive with period instruments, and his pictorial, dance-infused writing profits from McGegan's singular rhythmic zest -- a sort of happy contagion.

Cooley calmly shatters tenorial stereotypes. His supple, multicolored voice is deployed with the utmost taste and delicacy. He was at his lyrical best in the winsome "Where'er you walk," from Handel's "Semele" -- a signature piece of the legendary Irish tenor John McCormack. Cooley has nothing to fear from the comparison.

Thursday's closer, Haydn's Symphony No. 8 ("Evening"), was a meeting of the impish, its surprises magnified by McGegan at every turn. Blending elements of the concerto grosso, divertimento and symphony, the piece frames the virtuosity of individual players. First among equals was bassist Chris Brown, whose tipsy-sounding solos in the Minuet were sheer delight.

Larry Fuchsberg writes frequently about music.