In both replantation and transplantation, circulation is restored to ischemic muscle by vascular anastomoses. As a result, the length of time that skeletal muscle will endure ischemia is important to know clinically. Using microsurgical techniques, we shut off the blood supply to the quadriceps femoris muscle in adult Wistar rats--by occluding the artery, the vein, or both for varying time intervals. The muscle was maintained in the animal for 48 hours, and then examined histologically. Our results indicate that the period of warm ischemia should be limited to about one hour, because a longer period of arterial ischemia produces severe histological changes in muscle. The limiting factor seems to be the arterial, rather than the venous supply. The possibility of muscle regeneration after such ischemic insults was not investigated.