The posterior semicircular canal of the frog was sinusoidally stimulated by mechanical endolympathic flow. Interaction of the excitatory and the inhibitory responses was analyzed on the spike density histogram. When the oscillation started in ampullofugal direction, the evoked action potentials were readily suppressed by the following ampullopetal stimulus. When the oscillation started in ampullopetal direction, the action potential appeared slowly, indicating that the inhibitory response has a longer time course than the excitatory response.