Lung components were analysed and dynamic pulmonary compliance was determined in 10 young healthy female subjects and seven adult male patients with bronchial obstruction. In normal subjects with a single ventilatory component (by multiple breath nitrogen washout method) a change of respiratory frequency did not affect dynamic lung compliance. Other normal subjects had two ventilatory components; in them and in the patients with bronchial obstruction, an increase of respiratory frequency decreased dynamic pulmonary compliance. A change of respiratory rate caused a greater change of dynamic lung compliance in the patients with bronchial obstruction than in normal subjects with two-component lungs. The results indicate that frequency-dependence of compliance and non-uniform distribution of inspired gas are caused by a similar mechanism. Inequality of regional time constants may be an important factor in this mechanism. The data also show that a decrease of dynamic lung compliance by more than 20% at a respiratory rate of 80 to 100 breaths/minute may be indicative of lung disease with obstruction.