It is known that sympatho-adrenal control of airways is increased in asthma since beta blockade can cause severe bronchoconstriction in asthmatic individuals. It has not been established whether an altered catecholamine response to exercise plays any part in the production of the common symptom of exercise-induced asthma (EIA). We have investigated this indirectly by measuring arterial plasma cyclic nucleotide levels in 10 subjects with EIA and five normal subjects. Cyclic AMP, which in this context reflects beta stimulation, rose significantly by 25.4% in the normal subjects during exercise, while there was no significant change during or after exercise (less than 5%) in the asthmatic subjects. Cyclic GMP rose significantly after exercise in the asthmatic subjects. Six normal subjects repeated the protocol before and after inhalation of salbutamol aerosol, 1600 microgram daily for 18 days. This did not reduce the cAMP response to exercise, and we conclude that the diminished cAMP response of the asthmatic subjects was not caused by their medication. The results may indicate either impaired catecholamine production or endogenous beta receptor hyporesponsiveness in some asthmatic subjects and this may contribute to the development of EIA.