The effects of bronchodilator treatment (intravenous atropine or inhaled metaproterenol) on the behavior of pulmonary resistance (RL) and dynamic compliance (Cdyn) as functions of respiratory frequency were studied in patients with chronic obstructive pulmonary disease and in apparently normal smokers. No systematic changes in the frequency-dependent behavior of RL and Cdyn were observed after bronchodilator treatment; the degree of frequency dependence increased in some subjects, and decreased or remained unchanged in others. The present results suggest that the effects of bronchodilators on the relationships between lung mechanical properties and respiratory frequency are not based on a single mechanism, but probably reflect the interaction of multiple factors possibly associated with the bronchodilator treatment.