The renin-angiotensin-aldosterone system was studied in patients with renovascular hypertension. When a liberal sodium diet was used, aldosterone excretion rate was increased in four of 14 patients with benign hypertension, who subsequently improved postoperatively. Aldosterone excretion rate was measured in six patients following surgery, and the values were lower than preoperative values in all six. Aldosterone secretion rate tended to be slightly higher in patients who improved postoperatively than in those who did not improve. Both the excretion and secretion rates were elevated in all five patients with malignant hypertension. The level of arterial angiotensin was normal in all patients with renovascular hypertension in whom it was measured. These findings indicate that if the activity of the renin-angiotensin-aldosterone system is increased, this increase is subtle and difficult to demonstrate. Consequently it is unlikely that the blood pressure elevation in patients with benign renovascular hypertension can be explained simply in terms