The purpose of this study was to evaluate the oxidative capacities and the rate of energy synthesis in isolated mitochondria extracted from normal and post-ischemic myocardium. Isolated rat hearts were perfused according to the working mode with a Krebs Heinseleit buffer containing glucose (11 mM), insulin (10 IU/l) and caprylic acid (25 microM). After a 15 min perfusion in normoxic conditions, the hearts were subjected to a 20 min local zero-flow ischemia followed by a 20 min reperfusion. During the perfusion, the aortic and coronary flows, the aortic pressure and the electrocardiogram were monitored. At the end of the reperfusion period, the non-ischemic and ischemic zones (NIZ and IZ, respectively) were separated and the mitochondria were harvested from each zone. The oxygen uptake and the rate of energy production of the NIZ and IZ mitochondria were then assessed with palmitoylcarnitine as substrate in 2 buffers differing in their free calcium concentration (0.041 and 0.150 microM). Ischemia provoked a 50% reduction of coronary and aortic flows. The reperfusion of the IZ allowed the partial recovery of coronary flow, but the aortic flow decreased beneath its ischemic value because of the occurrence of severe arrhythmias, stunning and probably hibernation. The IZ mitochondria displayed a lower rate of oxygen consumption, whatever the buffer free calcium concentration. Conversely, their rate of energy production was increased, indicating that their metabolic efficiency was improved as compared to NIZ mitochondria. This might be due to the mitochondrial calcium overload persisting during reperfusion, to the activation of the inner membrane Na+/Ca2+ exchange and to a significant mitochondrial swelling. On the other hand, the presence of an elevated free calcium concentration in the respiration buffer provoked some energy wasting characterized by a constant AMP production. This was attributed to some accumulation of acetate and the activation of the energy-consuming acetylCoA synthetase. In conclusion, ischemia and reperfusion did not alter the membrane integrity of the mitochondria but improved their metabolic efficiency. Nevertheless, these in vitro results can not reflect the mitochondrial function in the reperfused myocardium. The mitochondrial calcium overload reported to last during reperfusion in the cardiomyocytes might mimic the free calcium-induced reduction of metabolic efficiency observed in vitro in the present study. The resulting energy wasting might be responsible for the contractile abnormalities noticed in the reperfused myocardium.