Plasma concentrations of vasopressin (antidiuretic hormone), a powerful pressor agent and primary regulator of body fluid homeostasis, are elevated in a dose-related fashion to increases in exercise intensity. The response of plasma vasopressin levels to exercise is not modified by gender, but is reduced with training at absolute submaximal exercise intensities. However, the duration of the exercise and hydration status of the subject alters the observed increase. The increase in plasma vasopressin concentrations during exercise may be mediated by a variety of factors: plasma osmolality, blood pressure, blood volume, plasma concentrations of angiotensin II, psychological variables, and peripheral nerve stimulation. Furthermore, decreases in the metabolism of vasopressin, due to alterations in renal and hepatic blood flow during exercise, may contribute to the increase in plasma concentrations. Although plasma vasopressin concentrations are elevated during exercise, there is no definitive effect during exercise, as the excretion rate of free water is increased, sweat rate and composition are not changed, and a role in plasma volume regulation is dubious. Thus, the function of the elevation in plasma vasopressin levels during exercise is unclear at the present time.