An inhibitor of extraneuronal uptake, deoxycorticosterone acetate (DOCA), increased the sensitivities of the rabbit ear artery to extraluminal and to intraluminal epinephrine, each approximately threefold. The effect on extraluminal epinephrine was increased in cocaine-treated or chronic sympathetic denervated arteries. That the sensitising actions of DOCA were due to inhibition of extraneuronal uptake was suggested (1) by the relative magnitudes of its sensitising effects on the three amines [in decreasing order, epinephrine greater than isoprenaline (beta-stimulation) greater than norepinephrine] and (2) by its ability to prevent the potentiation of epinephrine by normetanephrine. That the uptake system was readily saturated was suggested by the observation that the sensitising actions of DOCA were decreased by procedures which decreased the potencies of epinephrine and isoprenaline, namely, phentolamine, and doses of isoprenaline eliciting constriction via alpha-stimulation, respectively.