Studies were performed to investigate the nature of the hyperkalemia and hypokaliuria observed in two patients after renal allotransplantation in both of whom the graft was functioning and urinary output was adequate. In the first patient, examinations in the upright position revealed low values for plasma renin activity and plasma aldosterone. The second patient showed similar findings, and, in addition, an angiotensin II (Hypertensin) infusion was ineffective in stimulating aldosterone secretion during the hyperkalemic state. In both patients, short-term administration of 9 alpha-fluorohydrocortisone caused kaliuresis with a definite decrease in serum potassium. There was a spontaneous correction of the biochemical and hormonal abnormalities six weeks after transplantation and the response to an angiotensin II infusion normalized. We suggest that the hyperkalemia in both of these patients was due primarily to the transient failure of the renin-angiotensin system of the transplanted kidney.