The influence of cardiac stunning on the oxidation of fatty acids and the oxidative phosphorylation in mitochondria was investigated. Rat hearts were perfused for 15 min according to the working mode with a Krebs-Henseleit buffer containing glucose (11 mM). The hearts were then maintained in normoxic conditions (C group) or subjected to a 15-min global no-flow normothermic ischemia followed by a 30-min reperfusion (R group). Throughout the perfusion, the aortic and coronary flows, and the heart rate and oxygen consumption were monitored. At the end of the perfusion procedure, a bolus of 1-14C palmitate was injected in the coronary arterial bed to evaluate the fatty acid oxidation. Two sub-populations of mitochondria were isolated from each heart by either mechanical (ME mitochondria) or enzymic (EE mitochondria) extraction and their respiration properties were evaluated. Furthermore, the mitochondrial energy production (ATP and creatine phosphate) was assessed. During ischemia, the aortic flow was suppressed and recovered only to approximately 50% of the preischemic value during reperfusion. This mechanical stunning was associated with an important reduction of the stroke volume (–37%,p<0.01) and="" a="" slight="" decrease="" in="" heart="" rate="">p<0.001). at="" the="" end="" of="" reperfusion,="" the="" beta-oxidation="" rate="" constituted="" 55±1.7%="" of="" the="" cell="" palmitate="" and="" was="" similar="" to="" that="" assessed="" in="" the="" c="" group.="" the="" oxygen="" consumption="" was="" decreased="" to="">μL O2/min/gww and the venous O2 concentration increased to 5.1±0.572 μL O2/mL (instead of 2.9±0.342 μL O2/mL in the C group), although due to large SD, only the latter was statistically significant. A decrease in metabolic effeciency (42±14.4 vs 106±16.8 mL/μL O2 in the C group) and an increase in palmitate oxidation to oxygen consumption ratio (77±10.1 vs 47.6±4.25 % beta-oxidized palmitate/μL O2 in the C group) were observed. This increased fatty acid contribution in the oxidation metabolism could be responsible for some oxygen wasting and could contribute to decrease the energy available for the contraction despite the normal cardiac oxygen uptake. Furthermore, the respiration parameters of the mitochondria were similar in the C and R groups when glutamate (20 mM) or palmitoylcarnitine (25 μM) were used as substrate. ME mitochondria of R group displayed a reduced rate of ATP production (118±29.5 vs 180±14.5 nmoles/min/mg proteins in the C group) without altered creatine phosphate production. The presence of calcium in the medium (10–5 M) provoked a decrease in ATP production. These effects were not observed with EE mitochondria. Thus, a decreased energy production resulting from a substrate effect and/or a decreased mitochondrial phosphorylative capacity could be associated with the mechanical stunning.