The purpose of this study was to compare exercise-induced changes in urinary cyclic AMP and cyclic GMP of normal and asthmatic subjects. All subjects were exercised to 80% of predicted maximum heart rate for 15 minutes on a bicycle ergometer. FEV1 was measured before and after exercise to determine the degree of bronchoconstriction due to exercise. Five urine specimens were collected by voluntary voiding at 20 minute intervals (one specimen before exercise and four after exercise). Before exercise, the asthmatic subjects excreted less cyclic GMP (p less than 0.05) but the same cyclic AMP as the normal subjects. Creatinine excretion in nine asthmatic subjects decreased by 24% (p less than 0.01) during exercise. In sixteen normal subjects (post-exercise bronchoconstriction less than 10%) cyclic AMP excretion increased by 19% (p less than 0.05) within 20 minutes after exercise, while cyclic GMP excretion did not change significantly. In thirteen asthmatic subjects (post-exercise bronchoconstriction greater than 20%) cyclic AMP excretion did not change significantly, while cyclic GMP excretion increased by 23% (p less than 0.05) within 20 minutes after exercise. This study has shown that exercise affects the excretion of cyclic AMP, cyclic GMP, and creatinine in normal and asthmatic subjects. The response of normal and asthmatic groups to exercise was the same for creatinine but different for cyclic AMP and cyclic GMP.