The effects of catecholamines and other sympathomimetic amines with alpha adrenergic activity on vasoconstriction were studied. Short ring segments were prepared from five rabbit blood vessels including the mesoduodenal, brachial, central ear and pulmonary lobar arteries and the saphenous vein. Constriction was elicited by electrical field stimulation of adrenergic neurons. This response was markedly potentiated in the mesenteric preparation by low concentrations of norepinephrine, epinephrine, phenylephrine, methoxamine, naphazoline and oxymetazoline. The potentiation occurred with amine concentrations not sufficient to elevate the spontaneous transmitter release or basal smooth muscle tone. Propranolol, cocaine and metanephrine did not prevent the potentiation. Isoproterenol and dopamine potentiated the response only in high concentrations and tyrosine was without an effect. An unidentified extraneuronal action was probably responsible for the potentiation. The mesenteric artery was unique for its marked potentiation as the brachial artery was potentiated only slightly and the other three vessels were inhibited by the alpha adrenergic agents in the constrictor response to stimulation.