The effect of sympathetic denervation on the sensitivity of the rabbit ear and basilar arteries and the cat middle cerebral artery to vasoactive agents was examined in relation to the functional significance of the innervation. Fourteen days after superior cervical ganglionectomy, the catecholamine fluorescence disappeared and the norepinephrine content drastically decreased and transmural nerve stimulation ceased to elicit any response in all these arteries. In the rabbit ear artery, denervation resulted in a significant leftward shift of dose-response curves for norepinephrine (8.2-fold), serotonin (7.7-fold), histamine (2.9-fold) and potassium (1.2-fold). In contrast, there was no significant shift of dose-response curves to these drugs in the rabbit basilar and cat middle cerebral arteries. These results indicate that after chronic superior cervical ganglionectomy, postjunctional supersensitivity developed in the rabbit ear artery, but no pre- or postjunctional supersensitivity occurred in the rabbit basilar and cat middle cerebral arteries. The difference may be in part attributable to the reported closer synaptic cleft distance of the sympathetic nerve terminals in the ear than in the cerebral arteries examined. These results further suggest that the sympathetic adrenergic nerve exerts a tonic influence over the rabbit ear artery, but little or none in normal cerebral blood vessels.