An electrophysiological analysis of the blink reflex (BR) was undertaken in 35 normal subjects and in 34 patients with Parkinson's disease. The blink reflexes were elicited by unilateral percutaneous electrical stimulation of the supraorbital nerve.The electromyographic (EMG) activity of the early (Rl) and the late (R2) components of the blink.reflex response was bilaterally recorded from the orbicularis oculi muscle using surface electrodes. Latencies, amplitudes, reflex duration and reflex areas of the earlv and the late comoonents were analyzed. In addition, a habituation index for each component was calculated comparing the EMG activity of that component -quantified by means of rectification, averaging (8 trials) and integration- at high (1 Hz) and low rate (0.1 Hz) stimulation. An. increase in the habituation index (decreased habituation) of the ipsilateral and contralateral R2 components was the most evident finding in Parkinson's disease patients. The coexistence of a normal Rl habituation index in these patients supports the hypothesis that in Parkinson's disease the underlying pathophysiology involves interneuro:;s ra:her than motoneuron. It was also found .that the EMG activity (Amplitude, Area) of both Rl and R2 components was lower than that in normal subjects, while the latency and the duration of these components were mainly normal in Parkinson's disease patients. The abnormality of the blink reflex parameters were less prominent in patients receiving L-dopa therapy. No correlation was noted between the blink reflex parameters and the clinical parameters in Parkinson's disease patients.