Abstract
Plasma arginine vasopressin concentrations were measured in 5 healthy volunteers during postural change under conditions of dehydration and normal hydration. A rise in plasma arginine vasopressin was observed only after dehydration and standing for 40 min. Five other volunteers who developed presyncopal symptoms during orthostasis had exceedingly high plasma arginine vasopressin levels. Changes in plasma arginine vasopressin concentration occurred with no significant alterations in plasma osmolality.