1. Venous plasma histamine was measured by a specific and sensitive radioenzymatic assay in seven male extrinsic asthmatic and six age-matched non-atopic non-asthmatic male subjects during exercise and voluntary isocapnic hyperventilation.
2. There was no change in peak expiratory flow in normal subjects with exercise or hyperventilation, but asthmatic subjects showed a 29.4 ± sem 5.8% fall after exercise and a 29.0 ± 5.4% fall after matched hyperventilation.
3. Plasma histamine was significantly higher (P < 0.05) in asthmatic (6.2 ± 0.95 nmol/l) than that in normal subjects (3.4 ± 0.61 mmol/l) and showed a significant (P < 0.01) rise (to 14.4 ± 1.83 nmol/l) during exercise in asthmatic, but not in normal subjects. This suggests that discharge of mast-cell mediators may occur during exercise in asthmatic subjects who develop exercise-induced asthma.
4. With hyperventilation there was no change in plasma histamine in either asthmatic or normal subjects, but this does not exclude the possibility that mediators may be released locally in the airways.