Renal resistance vessels of the mature spontaneously hypertensive rat (SHR) exhibit an increased reactivity to exogenous norepinephrine, but a normal response to renal nerve stimulation. This difference could be due either to depression of the exocytotic process or to accelerated disposition of the released transmitter. We compared the overflow of norepinephrine in isolated perfused kidneys from adult SHR and normotensive rats. After previous incubation with 3H-norepinephrine, renal nerve stimulation caused smaller increases in the overflow of intact tritiated transmitter and its metabolites in kidneys form SHR than in those from normotensive controls. A similar difference was found when the amounts of endogenous norepinephrine were measured radioenzymatically. The tissue content of norepinephrine was comparable in kidneys from both hypertensive and normotensive animals. The uptake of 3H-norepinephrine was comparable in kidneys from SHR and normotensive controls; cocaine caused a comparable depression of the 3H-uptake in which then explains the normal vasoconstrictor response to renal nerve stimulation despite the increased responsiveness of the vascular smooth muscle cells to norepinephrine.