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1. Plasma adrenaline was measured at rest and on exercise in six normal, non-atopic subjects. The exercise test produced a minimum tachycardia of 158 beats/min and a mean plasma adrenaline level of 1.89 nmol/l. 2. On a separate occasion, two doses of l-adrenaline (0.025 μg min−1 kg−1 and 0.05 μg min−1 kg−1) were infused in the same subjects at rest to produce two mean plasma levels similar to those found on exercise. 3. Significant bronchodilation and elevation of plasma glucose occurred with both infusion doses when compared with placebo. 4. Neither infusion caused a significant change in heart rate and only the higher dose caused a small but significant rise in systolic and fall in diastolic blood pressure. 5. Our results suggest that adrenaline as a hormone, at the levels found on exercise, is capable of causing bronchodilation and glycogenolysis in normal subjects. Its contribution to the cardiovascular response to exercise appears to be slight and suggests that its role in cardiovascular physiology may also be marginal.

1. 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. 2. During a second study, adrenaline was infused intravenously in six resting subjects at a rate of 0.025 μg 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. 3. 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. 4. 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. 5. There is no significant increase in secretion by the adrenal medulla in response to the stimulus of mild or moderate exercise.