The hemodynamic profile following administration of prazosin suggests that it may be a less effective antagonist of postsynaptic α-adrenoceptors in veins than in arteries. One possible explanation for this would be if there exist more than one postsynaptic α-adrenoceptor subtype and if these subtypes varied in their distribution between arteries and veins. We addressed this question by examining the extent of the antagonism by prazosin of norepinephrine-induced increases in vasomotor tone in several arteries and veins in vitro, including the thoracic aorta of the guinea pig and the portal vein, saphenous artery and vein, and the renal artery and vein of the rabbit. Dose-response curves were constructed following increasing concentrations of prazosin. Based on these dose-response curves, and analysis of the various dose ratios using the Schild plot, we conclude that prazosin displays the characteristics of an equilibrium competitive antagonist at postsynaptic α-adrenoceptors in arteries and veins in vitro, and that prazosin displays little, if any, selectivity for arteries in comparison to veins.