For immunogenetic and economical reasons, an operative technique for lung transplantation in the rat was developed. With the use of an operation microscope and 8-0, 9-0, and 10-0 sutures, the left pulmonary artery, vein, and brochus were anastomosed. The mean operation time was 4 hours; the mean graft ischemia time, 87 minutes. After frequent failures initially, due to anesthetic problems and lack of experience with microsurgical operative technique, a final peroperative survival of about 80% was obtained. Postoperative mortality approximated 50%. The main cause of postoperative death was thrombus formation at the site of an anastomosis. In a final series of 28 isogeneic transplantations, proper data for a one-month follow-up was obtained from 36% of the animals that underwent operation, a percentage comparable with initial canine pulmonary and rat renal transplantation studies. Left lung transplantation in the rat proved to be technically feasible and may provide an immunogenetically well defined and economically advantageous animal model in lung transplantation research.