This study investigated the effect of moderate treadmill exercise on the airways of normal and asthmatic children using an effort-independent method (total respiratory resistance) as well as conventional pulmonary functions. Changes in total respiratory resistance (Rrs) were measured during and after exercise using a forced oscillation technique. Conventional spirometric (PEFR, FEV1 and MMEF) and body plethysmographic measurements (Raw, Vtg) were also determined before and after exercise. A rapid, signigicant decrease in Rrs, suggesting bronchodilation during exercise, was observed in both groups tested, but this decrease was greater in normal children than in asthmatic patients. The bronchodilation persisted throughout the exercise period in both the asthmatic and the normal children. Increasing the length of the exercise from 10 to 20 min did not alter the degree or the duration of exercise-induced bronchodilation. In both normal children and patients with bronchial asthma, during moderate treadmill exercice, a significant, persistent bronchodilation can be detected. At the end of exercise, bronchoconstriction developed in asthmatic patients, while the values in normal subjects returned to their preexercise levels.