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 (EC50); amount of maximum response (EAm), and affinity of antagonist to receptor (KB and pA2). The contractile response of the vessels to the various sympathomimetic amines involved alpha-adrenoceptors. Postjunctionally, the alpha 1 type of receptor predominated in the arteries whereas the alpha 2-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 F2 alpha.