1. Fourteen mildly hydropenic normal volunteers were slowly tilted at a constant rate from the horizontal to the 85° 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° 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.

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