This report describes a method using direct photon absorptiometry (DPA) for determination in vivo of the bone mineral content to bone width ratio (BMC/W) in the rat femur. A significant correlation was noted between measurements by DPA and whole femur dry weight and ash weight. Measurements were reproducible within repeated determinations of both the same and successive days. Soft tissue, rotation of the femur, or slight differences in the longitudinal positioning of the femur did not significantly affect usefulness of the measurements. A significant positive correlation was seen between body weight and the femur BMC/W, indicating that the method accurately reflected expected physiological changes in bone mineral. The method is suitable for the rapid, simple, accurate, and reproducible measurement of bone mineral in the anesthetized rat and is applicable to serial measurements.