The maximal concentrating ability of the clamped and the untouched kidney was studied in non-anesthetized Wistar rats which had been rendered hypertensive by the constriction of one renal artery. Both kidneys were able to elaborate a well-concentrated urine after dehydration and under the influence of exogenous vasopressin, whereby the clamped kidney was even superior in this respect to the untouched contralateral kidney. The concentration of inulin was regularly elevated in the urine from the clamped kidney. A slight depression of RPF and GFR ran parallel with a drop of blood pressure in the course of the experiment. Total sodium excretion in the Goldblatt rats was not different from that of normal rats; in hypertensive rats, however, two thirds of the total output were accomplished by the untouched kidney. The difference in concentrating ability between the two kidneys is interpreted as a consequence of different sodium reabsorption in the ascending limb of Henle’s loo