The effect of medication on peripheral airway obstruction was examined in cases of bronchial asthma. Subjects were 1) patients with exercise-induced asthma, 2) an animal model of hyperventilation-induced asthma and 3) patients with chronic asthma. Peripheral airway obstruction was induced in 30 of 51 patients with exercise-induced asthma. Induction of peripheral airway obstruction was protected significantly by procaterol. Cromoglycate was effective in 12 of 17 patients but ipratropium was not effective against induction of peripheral airway obstruction. In the animal model, humid air inhalation, procaterol and ipratropium completely prevented hyperventilation-induced bronchoconstriction, but cromoglycate caused only partial prevention. In cases of chronic asthma with peripheral airway obstruction, beclomethasone inhalation reduced the symptom rating rapidly, but no changes were observed in pulmonary function and threshold of airway responsiveness. Cromoglycate was started in patients with chronic asthma who had been treated with beclomethasone. After cromoglycate administration, therapeutic rating decreased, but increased again after 8 weeks of cromoglycate therapy. Peripheral airway obstruction induced by exercise or hyperventilation could be prevented by adequate premedication, but chronic peripheral airway obstruction was difficult to treat.