The activity of motor cortical inhibitory circuits was studied with paired transcranial magnetic stimuli in 16 patients with Parkinson's disease 'off' therapy, five patients 'off' and 'on' therapy, and 11 normal subjects. Paired stimuli were delivered at short (3-20 ms) as well as long (100-250 ms) intervals during slight voluntary contraction. The intensity of the conditioning stimulus was subthreshold (80%) at short, and suprathreshold (150%) at long intervals. In addition, the silent period following a single magnetic shock given at 150% of threshold was measured. With short interstimulus intervals, no significant difference between patients and normal subjects could be detected. With long interstimulus intervals, the test response was significantly more inhibited in patients than in normal subjects. Although the cortical silent period was found to be slightly shorter, the recovery of motor evoked potentials was incomplete in patients with Parkinson's disease. This alteration could be partially reverted in dopaminergic therapy. In conclusion, the responsiveness of motor cortices to suprathreshold magnetic stimuli delivered after the end of the silent period is impaired in patients with Parkinson's disease, possibly due to prolonged activity in intracortical inhibitory circuits. The positive effect of L-dopa suggests that dopaminergic modulation of cortical activity, most probably at basal ganglia level, is involved in the pathogenesis of this phenomenon.