Data from recent studies in experimental animals suggest that central dopaminergic mechanisms may be involved in regulation of the regional cerebral blood flow (rCBF) in the mammalian brain. In order to evaluate possible effects of dopaminergic stimulation on human cerebral circulation, rCBF was measured by the 133Xenon inhalation technique in 26 patients with Parkinson's disease before and after the chronic oral administration of levodopa combined with a peripheral Dopa-decarboxylase inhibitor. Treatment with levodopa did not induce significant alterations in the mean brain, mean hemispheric or regional flow values in these patients. These findings suggest that the dopaminergic system does not play an important role in modulation of the rCBF in man.