The effects of i.v. infusions of 3 and 6 micrograms/kg/min of dopamine (DA) and epinine on heart rate, arterial blood pressure, regional blood flows and vascular resistances in the renal, mesenteric and femoral vascular beds were compared in pentobarbital-anesthetized dogs. At the 3 micrograms/kg/min infusion rate, neither DA nor epinine changed blood pressure, whereas at the higher infusion rate both increased blood pressure by about 20 mm Hg. DA increased renal blood flow significantly at both infusion rates; whereas, epinine did not change renal blood flow. After administration of phenoxybenzamine, both epinine and DA decreased blood pressure; upon adding propranolol, the vasodepressor effect of epinine, but not of DA, was abolished. However, propranolol did not inhibit epinine-mediated vasodilation in the renal or mesenteric vascular beds, but a marked increase in femoral vascular resistance was observed. The addition of (R)-sulpiride, a DA antagonist, abolished DA and epinine-induced vasodilation in the mesenteric and renal vascular beds. Experiments in animals treated with hexamethonium to block ganglion transmission and propranolol to block beta adrenoceptors revealed that both selective alpha-1 (terazosin) and alpha-2 (rauwolscine) adrenoceptor antagonists inhibited the vasopressor response to DA to a greater degree than the responses to epinine. Thus, although DA and epinine possess significant DA1 activity, the consistent increase in renal blood flow observed with DA is not seen with epinine because of the more potent alpha adrenoceptor activity of the latter, which is mediated by both alpha-1 and alpha-2 adrenoceptors.