Recovery of the lungs after the reinflation of chronic atelectasis was studied experimentally using adult dogs. Obstructive absorption atelectasis was induced by banding the left main bronchus with a thin metallic plate. Tests were conducted 1 month after reinflation of the lungs. In the group of dogs which had suffered atelectasis for 3 months, static compliance and pulmonary blood flow were decreased by half at 1 month after reinflation; the conditions of reduced Po₂, increased A-aDo₂, and increased shunt flow rate in the affected lung persistently continued, and histological findings also showed collapse of the majority of alveoli and dilatation of the peripheral bronchioli, while macroscopically the lungs appeared to be aerated. It was therefore assumed that further aeration of the alveolar areas was impossible. The pulmonary function of the dogs within 1 month of atelectasis was restored to the same degree as that of the control group