Single photon emission computed tomography was used to measure regional cerebral blood flow in six normal subjects and 12 patients with Parkinson's disease, before and after acute oral administration of levodopa. The drug induced a significant increase in cerebral blood flow both in controls and patients. Before levodopa, there was no significant difference between the groups, either in flow values or in their pattern. The clinical effects of levodopa were not related to the hemodynamic changes. The results suggest that measuring the flow response to levodopa is not appropriate to demonstrate variations in central dopaminergic receptor sensitivity in man.