Studies from this laboratory have demonstrated that head-out immersion in isothermic water causes a cephalad redistribution of blood volume. The resultant central hypervolemia induces a marked natriuresis and diuresis and suppression of plasma renin activity, plasma aldosterone, and plasma arginine vasopressin. All of these changes are thought to be attributable to stimulation of cardiopulmonary receptors. Immersion also produces an augmentation of prostaglandin E (PGE) excretion, which reflects increased renal PGE synthesis. The ability of immersion in induce a prompt and profound central hypervolemia, without concomitant alterations in plasma composition, indicates that immersion might be a preferred investigative tool for assessing the effects of volume expansion on renal function and hormonal responsiveness in both normal individuals and patients with edematous disorders. In addition, this model constitutes an appropriate tool for simulating weightlessness.