To study the hypotensive effect of L-dopa in patients with Parkinson's disease on long-term L-dopa, the blood pressure (BP) was measured in 14 patients and 6 controls before 100 mg L-dopa plus decarboxylase inhibitor and for up to 180 min after dosing. Plasma monoamines and motor functions were assessed. The mean BP was reduced between 60 and 180 min after dosing in the patients, whereas such a reduction was not observed in 5 patients from whom L-dopa was withheld and in controls who showed a high ratio of plasma dopamine compared to plasma L-dopa after dosing. The L-dopa-induced reduction in BP was thought to be due to a central nervous mechanism.