Influence of presyncope and postural change upon plasma arginine vasopressing concentration in hydrated and dehydrated man.

Plasma arginine vasopressin concentrations were measured in five 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 further volunteers who developed presyncopal symptoms during orthostasis had exceedingly high plasma arginine vasopressin levels. Changes in plasma arginine vasopressin concentration occurred with no signficant alterations in plasma osmolality.