Impedance measurements of the respiratory system were performed by means of the technique of forced oscillations to study the mechanical characteristics of the respiratory system in 33 patients with chronic bronchitis without spirometric evidence of airway obstruction (FEV1 greater than or equal to 70% predicted). In 23 patients (69.7%) impedance was found to be abnormal. Inhalation of a beta 2-adrenergic drug (terbutaline) resulted in acute reversibility of these findings. In the 10 patients (30.3%) with a normal baseline respiratory impedance, isocapnic hyperventilation with cold air resulted in impedance characteristics similar to the abnormalities spontaneously occurring in the other group. These changes were found to be completely reversible after the inhalation of terbutaline. On the basis of these findings we conclude that impedance measurements of the respiratory system by means of the technique of forced oscillations allow the determination of mechanical airway abnormalities in patients with chronic bronchitis.