Plasma adrenaline and noradrenaline were measured in arterial blood and in forearm venous blood during isometric exercise and during a mental stress test. In both conditions arterial plasma adrenaline increased significantly, whereas arterial plasma noradrenaline remained unchanged. During isometric exercise the increase in plasma adrenaline was greater in venous blood from the exercising arm than from the resting arm. The extraction of adrenaline in the forearm was greater in the resting than in the exercising arm. Venous plasma noradrenaline showed a rebound phenomenon after isometric exercise and tended to decrease during the mental stress test. The results indicate that it is preferable to measure arterial concentrations of adrenaline as an indicator of sympathoadrenal activity rather than venous concentrations since the extraction of adrenaline in forearm might not be constant. It is suggested that a selective increase in arterial plasma adrenaline as opposed to an increase in both plasma adrenaline and noradrenaline may indicate a selective increase in sympathoadrenal activity in visceral organs.