1. Fourteen mildly hydropenic normal volunteers were slowly tilted at a constant rate from the horizontal to the 85 degrees head-up position in order to study the interrelationship between plasma arginine vasopressin concentration, plasma renin activity and the change of plasma volume. 2. Nine subjects did not develop vaso-vagal symptoms and were studied for 45-60 min. Arginine vasopressin rose biphasically in all subjects: a small initial rise, which was seen at 3 min and persisted for 30 min, was followed by a striking rise between 30 and 45 min, when the fall of plasma volume had reached its maximum (17%). 3. Plasma renin activity reached a maximum at 30 min but fell by 45 min, as plasma concentration of arginine vasopressin rose. 4. Five subjects developed vaso-vagal symptoms 4-24 min after reaching 85 degrees when the study was terminated. A striking increase of arginine vasopressin concentration was seen within 4 min of syncope, but there was no change of plasma osmolality, cortisol concentration or renin activity.