Response of renal blood flow to regional sympathetic nerve stimulation has been studied in anesthetized cats. When the sympathetic nerves to the kidney are stimulated at frequencies within the ‘physiological range’, renal blood flow first decreases markedly and then increases over the next 3 to 5 min to a value slightly below the control level. A similar ‘escape’ of the resistance response is found when the renal nerves are stimulated under conditions of constant flow. If the frequency of renal nerve stimulation is gradually increased the onset of the escape is greatly delayed, indicating that failure of impulse transmission or severe depletion of transmitter is unlikely to be the cause of the escape phenomenon. Intraarterial infusion of noradrenaline produces increased flow resistance with little or no subsequent escape showing that the escape cannot be attributed to loss of responsiveness of the vascular smooth muscle to the adrenergic transmitter. A quantitative decrease in the amount of transmitter released per impulse and intrarenal vascular control mechanisms related to blood flow autoregulation are discussed as possible causes of the escape phenomenon.