It has been reported that metformin reduces blood pressure although the mechanisms have not been described. Indeed, several mechanisms could be implicated including the interaction with α-adrenoceptors or inhibition of sympathetic outflow. Therefore, this study was designed to determine the capability of metformin to block the vasopressor responses induced by α1/2-adrenoceptor agonists or selective electrical stimulation of sympathetic outflow. For this purpose, Wistar male rats were anesthetized, pithed and cannulated for selective preganglionic stimulation of the vasopressor sympathetic outflow or drugs administration. The effect of i.v. bolus injection of metformin (180 and 310 mg/kg) or its vehicle (bidistilled water) was studied on the vasopressor responses induced by: (1) selective sympathetic stimulation (0.03–3 Hz); (2) exogenous noradrenaline (0.03–3 μg/kg); (3) methoxamine (1–100 μg/kg); and (4) UK 14,304 (0.1–30 μg/kg). The tachycardic responses to noradrenaline were also investigated in presence of metformin. The vasopressor responses induced by selective electrical stimulation of sympathetic outflow were diminished by metformin (180 and 310 mg/kg) and remained unchanged in presence of vehicle. Moreover, the vasopressor responses induced by exogenous noradrenaline, methoxamine and UK 14,304 were dose-dependently inhibited by i.v. bolus injections of metformin (180 and 310 mg/kg) and were not affected by vehicle. Metformin practically did not block the tachycardic responses to noradrenaline except at the dose of 3 μg/kg. Taken together, these results demonstrate that metformin is capable to block vascular α1/2-adrenoceptors but not cardiac β-adrenoceptors. Thus, this mechanism could contribute, at least in part, on the hypotensive responses induced by metformin.