Habituation of the human vestibulo-ocular reflex by prolonged low-frequency (0.01 and 0.005 Hz) harmonic (sinusoidal) angular acceleration was assessed by measuring the phase advance of slow-phase eye velocity at 0.01 Hz and the time constant of slow-phase eye velocity decay after impulses of angular acceleration. A single half-hour test session produced increases in low-frequency phase advance and decreases in time constant values in all 16 normal subjects. Although all subjects showed increases in low-frequency advance from the beginning to the end of the initial test session, only a few maintained the increases between test sessions. These changes are qualitatively similar to but quantitatively less than those observed with similar testing in monkeys. For clinical testing, to minimize the effect of habituation during harmonic accelerations, the testing time at low frequencies (i.e., less than 0.1 Hz) should be as short as possible.