The effects of ischemia with hypothermic cardioplegic preservation were studied in a heterotopic isograft rat heart transplant model. Hearts with ischemic time periods of 0.5, 4, 8, and 12 hours (n = 5 to 7 transplants per ischemic time period) were analyzed for structural and functional changes 50 days after transplantation. Postoperative recovery was prolonged with increasing duration of preservation times, but all transplants functioned normally 20 minutes and 50 days after transplantation. Function of right ventricular strips from graft and normal hearts were compared in a muscle bath. No differences were observed between baseline or isoproterenol-stimulated myocardium from all grafts or normal hearts. Electron and light microscopy studies of right and left ventricle and ventricular septum samples demonstrated marked fibrosis at 4, 8, and 12 hours of preservation with no differences between the 4, 8, and 12 hour grafts. There were no other differences in other features examined, including necrotic, mitochondrial, I-band, contractility, nuclear, endothelial cell, or intracellular lipid changes. We conclude that (1) except for significant fibrosis, transplants have normal structure and function after 4, 8, and 12 hours of preservation, (2) in assessing the efficacy of hypothermic cardioplegic preservation, chronic structural changes are more sensitive than functional changes, and (3) hypothermic cardioplegic preservation is feasible up to 12 hours but fibrosis may be the ultimate limiting factor in man.