1. The renal vascular response to intravenously administered dopamine was assessed in normal man by selective renal arteriography and xenon washout. Infusion of 3 μg min−1 kg−1 induced renal vasodilatation with an increase in the cortical component of blood flow. Arterial blood pressure was not influenced and a systemic effect was not demonstrable. Lower doses did not induce a renal response. Increasing dosage raised arterial blood pressure and induced subjective symptoms, but did not result in a further increase in renal blood flow. 2. Renal vascular resistance increased with increasing age in the normal subjects. A significant inverse relationship was found between the initial vascular resistance and the renal vasodilator response to dopamine. It thus appears that the vascular effects of increasing age (nephrosclerosis) may limit the dilator response to dopamine. 3. It is concluded that dopamine is an effective renal cortical vasodilator when administered intravenously at doses which are free from other systemic cardiovascular effects. The dose-response relationship must be considered in attempts at reversal of conditions characterized by renal vasoconstriction.