The peak response and the duration of bronchodilator effect were measured after four inhalations at 20 min intervals of 250 micrograms of aerosolized terbutaline in 13 patients with bronchial asthma. Patients were initially studied after strict avoidance of all adrenergic agents for 2 wk; they were then studied at 2 wk intervals for 12 wk while receiving 500 micrograms of aerosolized terbutaline q.i.d. The initial testing (TI) and the testing at the end of 12 wk (TII) were double blind. Both peak response and duration of action decreased significantly between TI and TII. Cumulative dose-response curves showed that further improvement occurred after each inhalation during TI but that there was significant improvement only after the first inhalation during TII.