The function of external respiration was studied by way of a dynamic observation of 15 animals whose course was uneventful following autotransplantation of the left lung. The longest follow-up period was 4 years. The following studies were conducted: spirography, bronchospirography, functional tests with the right (intact) lung excluded from respiration, bronchopneumotachygraphy, electromyography of the intercostal muscles, and analyses of blood gases and acid-base balance. During the early hours and days after left lung autotransplantation, hyperventilation and gas exchange decrease were observed in the operated lung, along with arterial and venous hypoxemia and signs of metabolic acidosis. With the right (intact) lung excluded and only the autograft breathing, a considerable reduction of oxygen saturation of the peripheral arterial blood was observed, which indicated the low compensatory capacity of the autotransplanted lung. Restoration and normalization of ventilation and gas exchange, blood gases, and acid-base balance were noted within 8 days after left lung autotransplantation. The compensatory capacity of the lung autograft, as shown by the test with the right (intact) lung excluded, was restored within the first 2 months after surgery. Following lung autotransplantation, in the absence of the Hering-Breuer reflex, the adaptative reactions of the external respiration apparatus are provided by the reflexes from the proprioreceptors of the breathing muscles of the thoracic wall and by humoral factors that acquire a leading role in the regulation of respiratory mechanisms. After removal of the right (intact) lung in the late postoperative period, the left lung autograft supports normal vital functions of the body (with the longest follow-up being 3 1/4 years). This convincingly proves that the organ's function and structure have been restored.