By studying six normal subjects during graduated treadmill exercise, we have confirmed that there is very little rise in venous plasma adrenaline levels during mild or moderate exercise. During a second study, adrenaline was infused intravenously in six resting subjects at a rate of 0.025 micrograms min-1 kg-1. This elevated the basal plasma adrenaline level from 0.28 +/- 0.04 nmol/l to 0.92 +/- 0.16, 1.16 +/- 0.20 and 1.28 +/- 0.19 nmol/l at 3, 5 and 7 min after the start of the infusion. The same adrenaline infusion was repeated in the same subjects 7 min after they started moderate exercise at a constant rate on a static exercise bicycle. Just before the start of the infusion, 7 min after the onset of exercise, plasma adrenaline had risen to 0.36 +/- 0.07 nmol/l. This rose to 1.86 +/- 0.30, 1.98 +/- 0.26 and 2.19 +/- 0.29 nmol/l at 3, 5 and 7 min after the start of this second infusion. Five minutes after the end of the infusion, while the subjects were still exercising, the mean level was 0.56 +/- 0.04 nmol/l. The venous plasma level of adrenaline is the result of a balance between the secretion of adrenaline by the adrenal medulla and the clearance of adrenaline from plasma. Our results suggest that the lack of any significant rise in plasma adrenaline during moderate exercise does not result from an accelerated clearance of adrenaline by exercising tissue. The clearance rate of adrenaline from plasma is reduced during exercise. There is no significant increase in secretion by the adrenal medulla in response to the stimulus of mild or moderate exercise.