The effect of ischaemia followed by reperfusion on energy metabolism was studied in human skeletal muscle after microsurgical free transfer. Muscle biopsy specimens from 11 patients treated by free muscle transfer for facial palsy, injury to an extremity, or scalp defect were studied. The biopsy specimens were taken during ischaemia and after one hour of reperfusion, respectively. They were analysed for ATP to uric acid and creatine phosphate by high pressure liquid chromatography. Ischaemia lasting one or two hours affected the energy metabolism of the muscle cell as evidenced by a 50% reduction in creatine phosphate; a 20% reduction in ATP and in the energy charge; a 100% increase in inosine monophosphate, and a 700% increase in hypoxanthine and xanthine. Reperfusion for one hour improved these figures somewhat, and induced the production of uric acid. Skeletal muscle can therefore tolerate ischaemia for up to two hours in the clinical situation without permanent damage to the tissues.