To assess the vability of canine hearts after prolonged ex-vivo heart perservation, we evaluated the myocardial metabolism by determining the high energy phosphate(HEP)levels and respiratory function of isolated mitochondria(MRF). Furthermore, we calculated the mitochondrial score(MS) for semi-quantitative analysis of ultrastructural changes. These three indicators were compared with the left ventricular function(LVF) following orthotopic heart transplantation(H-TX). Forty two hearts, harvested from mongrel dogs, each weighing 8.5kg～15kg, were divided into 3 groups : 24-hour immersion in modified Collins solusion(MC) (group A), 24-hour perfusion with MC(group B) and 24-hour perfusion with MC containing perfluorochemicals(PFC)(group C). After the 24-hour preservation, biopsies were obtained from 7 hearts in each group and HEP levels, MRF and MS were determined. The HEP levels, MRF and MS were significantly higher in group C than in group B and significantly higher in group B than group A. The cardiac function after H-TX revealed a similar tendency. These findings indicate that continuous hypothermic perfusion provides better protection than simple hypothermic immersion and that perfusion with an intracellular solution containing PFC preserves and protects an organ sufficiently before clinical heart transplantation.