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Experiments on 119 New Zealand rabbits were performed to measure healing strength of femurs. Four pairs of normal rabbit femurs were also studied in vitro by use of a four-point bending test to measure bending rigidity. Rabbit-sized octagonal cross-sectioned intramedullary rods with 12%, 20%, 50%, 75%, and 100% of femoral bending rigidity were specially prepared. For each animal, one intramedullary rod was implanted in an osteotomized femur through a standard lateral approach, with the other femur serving as a control. The results from the 12% rigidity rods were widely variable. In the 20% and 50% rigidity groups, the callus diameter was increased over the 75% rigidity group through all phases of healing. The torque ratio in this group increased up to eight weeks and then converged to the ratios of the other rigidities. The 75% rigidity group had shown increase of angular displacement up to eight weeks, while the 20% and 50% rigidity groups continued to improve and reached control strength at 12 weeks of treatment. Furthermore, the 20% and 50% groups absorbed more energy to failure in the later stage of healing than the 75% and 100% rigidity groups. Rods of 20% and 50% rigidity stimulate external callus formation and improve the stability of healing bone.