Effects of exogenous and endogenous noradrenaline, released by tyramine and lower body negative pressure (LBNP), on vascular post-synaptic alpha 1- and alpha 2-adrenoceptors have been compared in healthy volunteers. Intra-arterial (i.a.) infusions of noradrenaline and tyramine into the forearm were given in the presence of saline, of yohimbine and of doxazosin, and changes in forearm blood flow (FBF) were measured. Lower body negative pressure of -40 mmHg was applied without and with a continuous i.a. infusion of yohimbine and of doxazosin, and changes in FBF in both forearms were compared. Forearm blood flow was measured by venous occlusion plethysmography. Noradrenaline and tyramine reduced FBF dose-dependently and to the same extent. Both these vasoconstrictions were significantly reduced by yohimbine (P < 0.001 for both) as well as by doxazosin (P < 0.05 for noradrenaline and P < 0.001 for tyramine). The tyramine-induced vasoconstriction was more effectively reduced by doxazosin, whereas yohimbine reduced the noradrenaline-induced vasoconstriction more effectively. Lower body negative pressure reduced FBF in both forearms to the same extent. Doxazosin effectively inhibited the LBNP induced decrease in FBF (P < 0.05) whereas yohimbine had no effect (P > 0.05). These results are in accordance with a predominant intrasynaptic location of post-synaptic alpha 1-adrenoceptors and a predominant extrasynaptic location of post-synaptic alpha 2-adrenoceptors in human blood vessels.