The vasomotor response of dopamine and dopaminergic agonists was studied in vitro on middle cerebral arteries from cat and pial arteries from humans. The action of various inhibitors was tested in order to define the receptors involved. A contractile response could be obtained by epinine, apomorphine and dopamine in the mentioned order of potency. The effect was blocked by alpha-receptor as well as serotonin receptor antagonists. The mode of inhibition suggested that serotonin receptors rather than alpha-adrenoceptors mediated the dopamine-induced contraction. A dose-dependent dilatation could be evoked by the dopaminergic agonists on actively contracted pial arteries. The relative potency was epinine greater than dopamine greater than apomorphine. The order of potency for the agonists, together with blocking experiments (including a parallel shift in the log dose-response curve induced by bulbocapnine), indicated that the vasodilatation is mediated by specific dopamine receptors.