Experiments were performed to determine the conditions in which norepinephrine release from adrenergic nerve terminals in smooth muscle from canine blood vessels and spleen might be inhibited by prejunctional α-adrenergic receptor activation. Strips of aorta, mesenteric and splenic arteries, splenic capsule and portal and saphenous veins were labeled with 7-3H-norepinephrine and mounted for superfusion. In the portal vein, an inhibitory effect of prejunctional receptor activation with exogenous norepinephrine (1.2 × 10–6 M) on transmitter efflux could be demonstrated during electrical stimulation (9 V, 2 Hz) of the nerve terminals. By contrast, in the other tissues, inhibition of transmitter release during electrical stimulation or depolarization of the nerve terminals with K+ (40 mEq/l) could only be demonstrated after blockade of the neuronal uptake mechanism. That activation of prejunctional α-adrenergic receptors in blood vessels inhibits the exocytotic process is suggested by the failure of exogenous norepinephrine to affect either the basal efflux of 3H-norepinephrine or the displacement of 3H-norepinephrine by tyra