1. The effect of experimental myocardial infarction on exercise and recovery of rat skeletal muscle was studied using 31P n.m.r. 4 weeks post-operatively. 2. Myocardial infarction (12 +/- 3% of left ventricular volume), insufficient to produce haemodynamic manifestations of heart failure, was without significant effect on exercise bioenergetics of skeletal muscle. 3. Citrate synthase activity was reduced by 17% in the infarcted animals and there was a marked slowing of the rate of phosphocreatine recovery after infarction (half-time 0.7 +/- 0.1 min to 1.6 +/- 0.2 min) in the absence of evidence of left ventricular failure or hypertrophy. 4. The study of recovery bioenergetics could provide a more sensitive measure of mitochondrial function than exercise, where no bioenergetic abnormality was detected. 5. Myocardial infarction can produce evidence of mitochondrial abnormality in skeletal muscle in the absence of haemodynamic compromise.