The effects of sound on the electrically elicited blink reflex were examined in 11 PD patients and 8 control subjects. The blink reflex was conditioned by sound stimulation which did not produce the blink response by itself. Latencies of onset, durations and amplitudes of the R1 and R2 components of the blink reflex produced by single electrical stimulation in PD patients were not different from those in control subjects. In control subjects, sound stimulation produced the facilitation of the R1 component at intervals between 50 and 100 msec, the facilitation of the R2 component at intervals between -20 and 10 msec and the suppression of the R2 component at intervals between 100 and 1000 msec. In PD patients, the facilitation of the R1 and R2 components increased and the suppression of the R2 component decreased. The hyperfunction of the facilitatory interneurones and the hypofunction of the inhibitory interneurones, relating with the blink reflex, might play a role in the abnormal motor control system in the faces of PD patients. These abnormalities might be related to the disturbed function of the reticular formation and the basal ganglia.