The direct effects of adenosine 3',5'-cyclic monophosphate (cAMP) and dibutyryl adenosine 3',5'-cyclic monophosphate (D-cAMP) on renal blood flow were studied in dogs under renal perfusion with constant pressure. Cyclic AMP caused a marked phasic decrease in renal blood flow in doses more than 1 mg, while D-cAMP produced a slight increase in that in doses more than 10 mg. The vasoconstrictive effect of cAMP was not blocked by the pretreatment with phenoxybenzamine, but was markedly depressed during the infusion of aminophylline. On the other hand, the vasodilative effect of D-cAMP was not affected either by the pretreatment with propranolol or during the infusion of aminophylline. These findings indicate that cAMP and D-cAMP have an opposite direct effect on the renal artery, independently of the adrenergic receptors. However, there may be a possibility that cAMP produces its vasoconstrictive effect on the renal artery partly through the action of its metabolites.