Transmastoid galvanic stimulation was applied to five subjects while records were taken of the rectified and averaged EMG from the triceps brachii of both sides. In four subjects current pulses of 1.6 mA, lasting 10-100 ms, evoked an excitatory response with a latency of 30-35 ms from the side ipsilateral to the anode and inhibition appeared at 40 ms on the side ipsilateral to the cathode. In the fifth subject the stimulus evoked inhibition at 40 ms on both sides. Thresholds for both excitatory and inhibitory responses were between 0.6 and 1 mA. Control experiments excluded a possible cutaneous origin. These actions might therefore represent reflex responses elicited by activation of the vestibular systems.