THE RECENT development of a simple reliable method for measuring plasma renin activity, together with the availability of a safe procedure for obtaining samples of blood from both renal veins, has prompted us to assess the utility of such measurements in the diagnosis of surgically correctable renovascular hypertension. Previous studies of this type have been reported by McPhaul,1 Fitz,2 Kirkendall,3 and Ueda,4 and their associates. A series of 22 patients, diagnosed by conventional measures as having renovascular hypertension, were subjected to operative treatment. Proof of the diagnosis was considered to be established if there was unquestionable improvement in blood pressure after corrective surgery. This is a report of the measurements of renin activity in blood plasma specimens obtained from both renal veins preoperatively and, whenever possible, postoperatively. In addition, plasma renin activity in effluent blood from both kidneys was measured in 13 patients with "essential" hypertension, who were not subjected