Biopsies of muscle from ten patients and of muscle and nerve from three monkeys were obtained during periods of continuous ischemia and studied by light and electron microscopy and by histochemical means. Between the third and fourth hours of ischemia, swelling of the mitochondria was noted in monkey muscle. One week later the appearance was normal. In the monkey nerves degenerated myelin was seen one week later but appeared to be normal at 2 weeks. If after 3 hours the tourniquet was deflated for 30 minutes and then reapplied for 2 additional hours, no changes were found. Succinic dehyrogenase analysis was used to identify swelling of the mitochondria. No mitochondrial changes were seen in human muscle made ischemic by a tourniquet for 2 hours.