We have studied the effects of surgical procedures on the blood supply to the femoral head in adult dogs. The blood supply to normal adult canine femoral heads and osteoarthritic femoral heads was assessed by microvascular injection techniques and by measurement of the rate of blood flow by the hydrogen-washout technique. Circulation to the femoral head in the mature dog normally is dependent on retinacular vessels. Vascular anastomoses between the epiphysis and the metaphysis are generally not larger than capillary size. Reaming the femoral head does not devascularize the bone unless the retinacular vessels are disturbed. Stripping of the retinaculum, or combined reaming of the femoral head and stripping of the retinaculum, devascularized the femoral head in adult dogs with normal femoral heads. In the osteoarthritic hips, vascular anastomoses developed between the epiphysis and the metaphysis, so that stripping the retinaculum did not devascularize the femoral head. However, the rate of blood flow was decreased after combined reaming and retinacular stripping. Clinical Relevance: In the non-arthritic hip or in one with early arthritis, the retinacular vessels are of primary importance to circulation to the femoral head. Damage to these vessels during surgery will lead to osteonecrosis in a high percentage of patients. The formation of vascular anastomoses between the epiphysis and the metaphysis during the development of osteoarthritis may make the arthritic femoral head less vulnerable. However, care should be taken to preserve retinacular vessels, since in this study the rate of blood flow was decreased by reaming the femoral head and stripping the retinaculum.