Plasma noradrenaline, adrenaline, and cyclic 3'5' AMP (cAMP) were measured in seven asthmatic patients with known exercise-induced bronchospasm and six matched non-atopic control subjects during a standard treadmill exercise test and then during matched isocapnic hyperventilation. Normal subjects showed a 5.5 fold rise in noradrenaline and a 3.2 fold rise in adrenaline during exercise compared with a 2.1 fold rise in noradrenaline and no significant rise in adrenaline in asthmatics who all developed bronchoconstriction after exercise (mean fall in peak flow rate 28.4 +/- 5.8%). Plasma cAMP rose 1.4 fold in controls but showed no significant rise in asthmatics. This reduced sympatho-adrenal response to exercise in asthmatics is difficult to explain. The failure of circulating catecholamines to rise and stimulate beta adrenoceptors on the mast cell may facilitate the release of bronchoconstrictor mediators. Matched hyperventilation produced bronchospasm in asthmatics (mean fall in peak flow rate 29.0 +/- 4.4%) but no change in catecholamines in either group suggesting that circulating catecholamines have no direct role in exercise-induced bronchospasm but may play a permissive role via the mast cell.