The silent period after contralateral and ipsilateral transcranial magnetic brain stimulation was studied in patients with Parkinson's disease before and after dopaminergic and anticholinergic therapy; in normal subjects before and after L-dopa administration and in patients with drug-induced parkinsonism. In patients and normal subjects the silent period was also studied after peripheral nerve stimulation. The silent period after transcranial cortical stimulation was shorter in Parkinson's disease patients than in normal subjects. In patients with Parkinson's disease L-dopa prolonged the silent period after transcranial brain stimulation and after ipsilateral cortical stimulation. Biperiden prolonged the silent period after transcranial brain stimulation. In normal subjects, L-dopa produced similar but smaller changes. In the patients with drug-induced parkinsonism the silent period after transcranial magnetic stimulation was shorter than normal subjects. The peripheral silent period was similar in normal subjects and in patients and did not change after drug administration. In conclusion cortical silent period is abnormal in patients with Parkinson's disease and drug-induced parkinsonism. Dopaminergic drugs modulate the duration of the cortical silent periods in patients and in normal subjects, through mechanisms acting mainly at basal ganglia and possibly also directly at cortical level.