In order to determine the beneficial or detrimental effects of motion on fracture healing, rabbits femora were fractured surgically and each fracture was immobilized with a apecially designed external skeletal flxation device, which permitted a controlled spontaneous angulatory motion. in the first control group no motion was permitted; in the second group 5 degrees, and in the third group 10 degrees of angulatory motion was permitted. At 7th post-operative week, the bony union rates were 79%, 93%, and 54% In the first, second and third groups, respectively. Radiographically, the fractures were united most sufficiently in the second group. The ultimate bending load was signlficantly greater In the second group than the control but no significant differences were observed between the other groups. The ultimate bending stress and the modulus of elasticity were significantly greater ln the second group than the control and the third group, but no signlficant dlfferences were observed between the control and the third group. Fracture healing was significantly enhanced in the second group as compared to the control and the third group. It is suggested that a certain amount of motion at the fracture site enhances fracture healing, while excessive motion hinders it. Such enhancement seems to be related with the motion itself, and not wlth the muscle or jolnt function, since the rabbits Included in this study permitted the spontaneous full weight bearing which also permitted near normal muscle and joint function.