Ketamine (1.1 × 10−5to 3.7 × 10−4 M) potentiated catecholamine responses of rat anococcygeus muscle and rabbit aortain vitro. In the anococcygeus, potentiation was abolished by cocaine (2.9 × 10−5 M) pretreatment or by chemical sympathectomy using 6-hydroxydopamine (6-OHDA), but was unaffected by pretreatment with the extraneuronal uptake inhibitor cortisol (8.3 × 10−5 M), or the catechol-O-methyltransferase inhibitor tropolone (2.4 × 10−4 M). The action of ketamine mimicked the potentiating effect of cocaine on tyramine responses. In contrast, the potentiation by ketamine in rabbit aorta was unaffected by cocaine or 6-OHDA but was abolished by cortisol or tropolone; and ketamine potentiated tyramine responses, whereas cocaine inhibited them. Thus the mechanism of action by which ketamine produces potentiation of catecholamines in these two tissues is completely different. These results suggest that ketamine has the unusual ability to block neuronal and extraneuronal uptake and that the predominating mechanism will depend on the type of tissue examined and the morphology of its adrenergic innervation.