To elucidate the physiological mechanisms involved in the cardiovascular and renal responses to spaceflight, we have developed a ground-based primate model which uses lower body positive pressure (LBPP) to simulate the chronic central vascular expansion associated with weightlessness. Four male squirrel monkeys with chronically implanted arterial and venous catheters and the capacity for continuous urine collection were subjected to LBPP for 4 days. Onset of LBPP resulted in an immediate diuresis, natriuresis and kaliuresis and a significant fall in plasma aldosterone and potassium levels. By day 2 the level of natriuresis had decreased by half, while potassium excretion and plasma aldosterone values had returned to control levels despite the persistence of a significantly reduced plasma potassium concentration. We conclude that the low plasma potassium level appears not to stimulate a compensatory fall in plasma aldosterone because of the simultaneous presence of body volume contraction acting to raise aldosterone levels.