The left horizontal semicircular canal of the leopard frog (Rana pipiens) was exposed and the entire intact nerve was draped over a platinum-iridium hook for recording action potentials (a.p.'s). The preparation was then rotated sinusoidally in the horizontal plane at 0.3 Hz with varying angular displacements. The arm of the canal was then transected, and a fine micropipette ligated in place in one end of the arm. Artificial endolymph was pumped into and out of the canal at the same frequency, while the amplitude of the displacement was varied from sub-threshold to supra-threshold values as determined from the resulting a.p. train. By comparing the two a.p. trains, it was determined that the movement of endolymph in the canals was 5.3 x 10(-8) cm3/deg/s, and that the threshold displacement of the midpoint of the cupula was 7.6 x 10(-5) cm (0.76 micrometers).