Peripheral plasma renin activity (PRA), vena cava and renal veins PRA were measured in 85 patients; arterial PRA was also measured in several cases. 25 patients had essential hypertension or primary aldosteronism; whereas in 60 patients renal or renovascular abnormalities were present. Peripheral PRA was elevated in 17/30 patients with parenchimal or excretory tract diseases. The following ratios came from the results of vena cava, and renal veins PRA were calculated: Ra/Rc and Rc/P according to Stockigt et al. and V-A/A according to Vaughan et al. The ratios were calculated only when v. cava PRA was greater than 4 ng/ml/3 hrs. A good correlation was found between v. cava can arterial PRA. Among the 60 cases with renal hypertension, 26 underwent either revascularization or nephrectomy surgery. Eleven of the 15 patients operated on for renovascular diseases showed a significant reduction of blood pressure (greater than 30 mmHg); all had high peripheral PRA and 6 had significant Ra/Rc and Rc/P (respectively greater than or equal to 1,5 and less than or equal to 1,3). The same patients also had V-A/A of the affected kidney greater than or equal to 0,48 and 5 also V-A/A of the controlateral kidney less than or equal to 0,23, both values being significant for a unilateral renin secretion and controlateral suppression. Among the 4 patients who were unsuccessfully operated on, only 2 had elevated peripheral PRA and 1 had Ra/Rc and Va-A/A greater than normal, but not significant values of Rc/P and Vc-A/A. 11 patients with small kidney or other forms underwent surgery; among the 6 cases with satisfactory results, 5 had high peripheral PRA, 2 showed significant ratios Ra/Rc and 3 a significant V-A/A for both kidneys. Only 1 of the patients not cured by surgery had supernormal peripheral PRA and none had any significant ratio. Therefore both methods for elaborating data obtained from the measurement of PRA in renal veins seem to offer similar prognostic indications. The finding of significant ratios is an almost sure criterium for predicting a surgical cure of renal hypertension, whereas the cases where we may expect a failure present ratios which are not significant. However, patients of the latter group may sometimes have successful results at surgery too, which demonstrates that other mechanisms may also be involved in the pathogenesis of this form of hypertension.