1. Syncopal or near syncopal episodes have been observed in five subjects who stood or were tilted and in whom blood samples were being taken.
2. In all subjects bradycardia and hypotension developed before the onset of symptoms. Increases in plasma adrenaline concentrations occurred in all subjects, beginning before the faint. Changes in plasma noradrenaline concentrations were variable: in three subjects there was a marked fall and in the other two subjects an increase. Plasma vasopressin increased in all subjects.
3. Increases in plasma adrenaline may be contributing to the vasodilatation and arterial hypotension which occur during syncope.