The metabolic effects of -5degrees tilt were studied in eight normal individuals. Exposure to tilt for 24 h increased solution excretion and decreased plasma volume. Plasma renin activity and plasma aldosterone levels were not significantly different from supine values during the first 6 h of tilting, but were increased significantly at the end of the 24-h tilt period. Creatinine clearance and potassium balance were not affected by the tilt. These findings indicate that head-down tilt induces a sodium diuresis and stimulation of the renin-angiotensin-aldosterone system.