A sine wave current stimulus, applied between electrodes placed about one ear and an indifferent electrode, produced a cyclical sway predominantly in the coronal plane. In 12 normal subjects, stimulus frequency (0.1-0.6 Hz) and intensity (+/-0.05 - +/-1.5 mA) had no significant effect on the amplitude of the postural response, but phase lag increased from 0.7 rad at 0.1 Hz to 3.8 rad at 0.6 Hz. Stimulation at 0.3 Hz, +/-1.0 mA evoked detectable responses in all but one of 50 normal subjects. Seven patients with end organ lesions all gave positive responses with the same test procedure. Likewise 13 out of 16 patients with destruction of the VIII nerve also gave positive responses. In the other 3 patients the response on the side of the lesion was absent, as it was in another 3 individuals with lesions of the vestibular nuclei. It was concluded that the sway induced by a low frequency alternating current is the manifestation of the stimulation of structures within the CNS central to the vestibular nerve or its end organ.