The effect of autotransplantation of the left lung on respiratory regulation was studied in four dogs in pentobarbital anaesthesia. In each dog the electrical and mechanical activity of the inspiratory muscles was studied before and 4-6 weeks, 3, 6 and 12 months after the transplantation. No or very little change was found in comparisons of spontaneous respiration or response to inhalation of carbon dioxide, hypoxia or combined hypercapnia and hypoxia. When the airways were closed after inflation with varying volumes of air, the increase in number and frequency of the electromyographic impulses in the intercostal muscles and the diaphragm (compared with the preceding unobstructed breaths) was greater before than after the lung autotransplantation. The duration of the first respiratory cycle after airway closure increased progressively with the volume of inflation in intact dogs and in dogs with autotransplanted left lung, though the increase was approximately halved following autotransplantation.