Local cerebral blood flow was measured in anaesthetized beagle dogs by the hydrogen clearance method. Dopamine was administered as a continuous intravenous infusion of varying doses. The changes in local cerebral blood flow induced by dopamine were similar at the different locations; i.e., the caudate nucleus, thalamus, frontal and parietal cortex. Blood flow responded to dopamine in the following ways: low dose (less than 2 micrograms/kg/min): blood flow decreased or remained unchanged; moderate doses (2--6 micrograms/kg/min): blood flow increased at all electrodes; high doses (7--20 micrograms/kg/min): blood flow decreased once again. The decrease in blood flow could be inhibited by the alpha-adrenergic receptor antagonist phentolamine or by the serotonin receptor antagonist methysergide. This indicates that the constrictor effect of dopamine on cerebral blood vessels is mediated via alpha-adrenergic receptors as well as via serotonin receptors. The increase in cerebral flow could be inhibited by the dopamine receptor antagonist haloperidol, indicating vascular dopamine receptors in the brain with a dilating effect. When the vasoconstrictor activity of dopamine is blocked, the single response to a dopamine infusion is a blood flow increase. This might be beneficial in the clinical situation of symptomatic vasospasm.