Complete arrest of blood flow was induced for 4.5 hours in the left hindlimb of Wistar rats by a tourniquet applied proximally to the thigh. Histologically the periost of the tibial bone after 3 days showed marked hypertrophy and hyperplasia of the periosteal cells with an osteogen differentiation. This response is similar to the initial formation of external callus during fracture healing. Transient ischemia may be an important factor in initiating fracture healing.