The authors studied bronchial reactivity in children with cystic fibrosis and with deformative bronchitis. The reaction of the bronchi was studied both by inhalatory and exercise provocation. Inhalatory tests were monitored by means of a whole body plethysmograph. Free-range run was used for the exercise test, and the peak expiratory flow rate was measured. The described technique of inhalatory tests was accurate, safe, and not tiring for the patient. No distinct difference in bronchial reaction between the two groups of patients could be stated. The bronchial reaction to the inhalatory and exercise tests was stronger in both groups than in normals but not as strong as in asthmatics. The nature of bronchial hyperreactivity in those patients is still unknown. The inhalatory test seems to be more sensitive than the exercise test in detecting that hyperreactivity.