A study was carried out to determine whether an episode of exercise-induced asthma (EIA) is followed by a refractory period and to assess the factors that affect it. Nine children, 9 to 14 years of age, were studied during repeated, paired exercise tests on a treadmill. Respiratory function was assessed by measuring peak expiratory flow rate before and after exercise. Changes were related to measured uptake of O2 during exercise. When paired tests were at a constant metabolic load, the severity of EIA was relatively constant in the first test of each pair, but it diminished in the second tests as the interval between tests decreased. The ability to bronchoconstrict fully was recovered within 4 hours after a test. When tests at varying metabolic loads were followed after 30 min by tests at a constant load, the severity of EIA increased in proportion to the load in the first test of each pair. In the second tests it was inversely related to that which occurred in the first tests. These results were compatible with the suggestion that the development of EIA requires the release of a stored mediator or enzyme precursor.