We studied two spinal cord inhibitory mechanisms, recurrent (Renshaw) inhibition and reciprocal inhibition, in seven patients with asymmetric Parkinson's disease in order to determine their contribution to the pathogenesis of rigidity. Recurrent inhibition, studied in the leg, did not differ from that found in normal subjects. All three periods of reciprocal inhibition, studied in the forearm, were present but reduced in magnitude compared with those observed in normal subjects. The arms, whether more symptomatic or less symptomatic, gave similar results. The diminution of all three periods of reciprocal inhibition is similar to the findings in patients with dystonia and is apparently indicative of an abnormal supraspinal influence on spinal mechanisms in these two disorders of basal ganglia function.