A problem in modern, agile aircraft is the possibility of the pilot losing consciousness during manoeuvres which can impose forces (G) many times that of gravitational acceleration. There is a possibility that the next generation of agile aircraft will exceed the level of anti-G protection presently available and the possibility of pharmacological intervention had been considered as an additional means of raising G-tolerance. Arginine vasopressin plays an active role in maintaining blood pressure, particularly in hypovolaemic situations, by its powerful vasoconstrictive action and control of heart output. As a potential G protection agent AVP has the appeal of not appearing to produce hypertension. The human centrifuge has normally been used to investigate problems of acceleration protection, but because of the hostile environment and space restraints, it has been difficult to carry out systematic biochemical investigations and lower body negative pressure has been used to simulate the cardio-vascular stress of G. The level of plasma catecholamines and arginine vasopressin has been measured during different exposure levels to lower body negative pressure and following b blockade with Propranolol combined with lower body negative pressure. No correlation could be established between peripheral circulating plasma levels of catecholamines and arginine vasopressin for any of the experimental conditions. However, dramatic oscillations in plasma noradrenaline levels were seen and a distinct pattern of arginine vasopressin secretion was observed at presyncope. The role of arginine vasopressin during syncope was investigated and small increases in plasma levels of this vasoactive hormone appeared to play a protective role in some subjects. In subsequent experiments, a bolus of arginine vasopressin was injected prior to exposure to lower body negative pressure, but this did not appear to have any effect on tolerance to lower body negative pressure and in some subjects it was positively detrimental.