The correct baroque beat should imitate the heart more than the clock. And it was at the heart that the Smithsonian's "Messiah" was aimed last night at the National Presbyterian Church. Authenticity and excitement were both well served in this performance by the Smithsonian Chamber Players and the American Boychoir, sensitively led by James Weaver.

It was the 37-piece orchestra that brought to life Handel's warhorse, with few of the intonation problems that often plague players of ancient instruments. Countertenor Jeffrey Gall, a true find, sang "But who may abide" with a warm, almost womanly sound that filled out Handel's ornamentation generously; and chest tones uncharacteristic in a male but decidedly effective. His style was closer to Monteverdi's then to the later Baroque, but his distinctive sound blended wonderfully with alto Elvira Green's in ". . . how beautiful are the feet" as the evening's vocal fabric unfolded colorfully. Green herself, while not the most elegant Handel singer, used her lush voice to dramatic advantage, and a slight wobble below a B was seldom distracting.

Bass Leslie Guinn and tenor Charles Bressler sang idiomatically but with minimal ornamentation. The still, small voice of soprano Carol Bogard showed the white top F's and A's of a little boy, with a lovely free vibrato in the lower voice.

A Handelian choir should have the clarity of noon and the warmth of the midday sun. The all-male chorus recalled perhaps dawn, and the famed American Boychoir from Princeton lacked the diction expected from a small ensemble. Its alto section in particular was thin. Only the adult tenor and bass sections, trained by Norman Scribner, sang with assurance.