We studied the inhibition of median H-reflex by conditioning stimuli on the radial nerve in 14 normal controls, 6 patients with unilateral and 1 patient with predominantly left-sided Parkinson's disease. In normal controls, the electrophysiological studies were performed on their right hands, yet both hands were examined in patient group. In the controls, we identified three inhibitory phases, with maximal inhibition at conditioning-test intervals of 0 ms (41.66 +/- 4.73%), 20 ms (45.19 +/- 4.33%), and 100 ms (44.55 +/- 6.84%), respectively. In the less- or a- symptomatic side of the patient group, the inhibitory patterns are similar to those of the controls. However, in the symptomatic arms, loss of inhibition, or even mild potentiation, was observed in the third inhibitory phase. When the symptomatic and asymptomatic sides of patients were compared, in contrast to the striking phenomenon found between symptomatic side and the controls, no difference was observed in the third phase. The current results imply that, although no obvious rigidity can be detected on the asymptomatic sides, subtle functional corruption may have occurred within the contralateral basal ganglia in patients with unilateral Parkinson's disease. The remarkable change of the third phase on the symptomatic sides of patients suggests the perturbation of the polysynaptic long latency reflex pathway may somehow play a role in the rigidity pathogenesis.