Changes in the mechanical properties of the rat femur caused by intramedullary application of bone cement were studied. In one group of animals reaming of the medullary cavity was performed, and bone cement was injected. In a control group only reaming was performed. At various intervals from day 0 to day 180 following operation the mechanical characteristics of the bone were evaluated. Reaming of the medullary cavity provoked a temporarily reduced bending moment of the femur at 40 days after operation. Intramedullary bone cement caused no significant changes in the bending moment of the femora until day 180 after operation. At this time, the bending moment of the cemented femora was significantly greater than that of the contralateral bones. This may be attributed to more periosteal bone being deposited, as implantation of bone cement induced a significant increase in periosteal reaction. The elastic stiffness of the bone was not altered either by reaming or the implantation of bone cement. It is concluded that reaming and implantation of bone cement into the medullary cavity do not impair the mechanical properties of the femur in the young rat.