Total exchangeable sodium and blood volume increased during the first year's treatment of seven hypertensive patients with guanethidine. Creatinine clearance values fell over the same period.A similar change was seen in two patients receiving reserpine. It is suggested that the hemodynamic changes produced by hypotensive drugs are insufficient completely to explain the observations. A direct effect of guanethidine on sodium reabsorption in the kidney may also be involved.