Approximately 200 isolated human mesenteric arteries (diameter less than 1 mm) and veins (diameter less than 2 mm) were tested in vitro for their contractile and dilator responses to adrenergic agonists under standardized conditions. This allowed for quantitative estimation of various receptor characteristics: relative agonist potency; concentration for half maximum response (EC<sub>50</sub>); amount of maximum response (E<sub>Am</sub>) and affinity of antagonist to receptor (K<sub>B</sub> and pA<sub>2</sub>). The contractile response of the vessels to the various sympathomimetic amines involved α-adrenoceptors. Postjunctionally, the α<sub>1</sub> type of receptor predominated in the arteries whereas the α<sub>2</sub>-receptor subtype was mainly found in the veins. None of the amines produced any vasodilatory effects in the vessels tested after previous contraction with prostaglandin F<sub>2α</sub>.