The effect of primary vestibular disturbance on postural control was investigated in 11 normal subjects exposed to perturbation by bi-polar binaural galvanic stimulation of the vestibular nerve. The stimulus consisted of 30 s of sinusoidal galvanic stimulation at a frequencies of 0.2, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0 Hz, with a current of +/- 1 mA, the subject standing with open or closed eyes, the response evoked being recorded with a force platform. As compared with resting values, i.e. no stimuli, variance of lateral body sway was significantly greater at all frequencies tested in the closed eyes condition and at frequencies of 0.2, 0.5, 1.0, 3.0 and 4.0 Hz in the open eyes condition; using a high pass filter with a cut-off frequency of 0.1 Hz, variance of lateral body sway was significantly greater at frequencies 0.2, 0.3, 0.5, 1.0 and 2.0 Hz in the closed eyes condition and at frequencies 0.5 and 2.0 Hz in the open eyes condition. These findings suggest that in the lateral plane vestibular input affects and probably contributes to human postural control over a wider frequency range than suggested by findings in previous studies. The trends in sagittal body sway were similar. Moreover, the visual contribution appears to enable the subject to suppress vestibular input causing lateral body sway only in the lower frequency range (here at 0.2 and 0.3 Hz), but with regard to the concomitant sagittal body sway at a much wider range of frequencies (here at all frequencies tested except 1.5 Hz).(ABSTRACT TRUNCATED AT 250 WORDS)