(1) The isolated dog kidney was perfused at constant pressure in a bath with blood from an intact unrelated dog receiving an intravenous infusion of normal saline and vasopressin. (2) It was found that the increase in sodium excretion from the perfused kidney was related to the renal arterial perfusion pressure in that the higher the pressure the greater the increase in sodium excretion. (3) The increase in sodium excretion was not related to changes in creatinine clearance, direct renal blood flow, filtered sodium, potassium excretion and PAH extraction, or to the fall in packed cell volume. (4) There was a significant inverse correlation between the rise in sodium excretion and the fall in plasma protein concentration; the greatest rise in sodium excretion being associated with the least fall in plasma protein concentration. (5) It is suggested that the exaggerated sodium excretion of hypertensive subjects when given intravenous saline may in part be due to the direct effect of the raised arterial pressure on the kidney.