Summary In order to assess the importance of nerve regeneration to the maintenance of adequate respiration by the replanted lung, bronchospirometric and blood gas analyses were made in 7 dogs 6 to 18 months after replantation of the left lung. Contralateral pneumonectomy was performed in 5 of these animals. Histologic regeneration of anatomically normal nerve fibers was demonstrated 6 to 18 months after replantation. However, these nerve fibers were functionally inadequate as evidenced by the absence of the Hering-Breuer reflex in all replanted lungs and of the cough reflex in the 2 dogs so tested. Respiratory rate and oxygen consumption were found to be dependent on the chemoreflexes initiated by the altered pO 2 and pCO 2 of the circulating blood. The replanted lungs exhibited substantial functional capacity but were unable to maintain normal oxygen exchange 6 to 18 months after surgery.