Left lung autotransplantation was performed in mongrel dogs to examine the limitation of tolerance of the deflated lung against warm ischemia in a warm ischemic time (WIT) range of 60-360 minutes. The animals were divided into the 4 following groups according to the duration of WIT and the use of heparin: Group 1; WIT 60-120 min., heparin (-), Group 2; WIT 120-240 min., heparin (+), Group 3; WIT 240-360 min., heparin (+), and Group 4; WIT 240-360 min., heparin (-). All the Group 1 animals tolerated a right pulmonary artery ligation on the 6th postoperative day, but some of the Group 2 animals died immediately after the reimplantation due to pulmonary edema. All the Groups 3 and 4 animals died. The PaO2 values during the right pulmonary artery occlusion, immediately after the reimplantation, correlated well with the survival of the animals. The maximum tolerable WIT of the deflated dog lung was considered to be 120 minutes.