We studied the effects of passive motion on joint stiffness, muscle mass, bone density, and regional swelling after an intra-articular injury. Instrumentation was applied to the hindlimbs of thirty adolescent New Zealand White rabbits to allow either passive motion or immobilization of the ankle. The knee was immobilized by the locking together of Steinmann pins that had been placed within the medullary canals of the tibia and femur. An intra-articular injury was produced by drilling of the tibial pin through the ankle joint into the talus and subsequent withdrawal of the pin from the ankle joint. The rabbits were divided into five groups, and they received four, eight, twelve, sixteen, or twenty-four hours of passive motion each day during the three-week period of study. One ankle of each rabbit was moved through an arc of 90 to 170 degrees of dorsiflexion at one cycle per minute, while the contralateral ankle was immobilized in 100 degrees of dorsiflexion with an aluminum splint, which was fixed to the aluminum block that was used to stabilize the knee joint. We found that sixteen and twenty-four hours of passive motion prevented stiffness of the joint. Passive motion for shorter periods was ineffective, even harmful, and resulted in stiffness ratios that were as much as four times higher than those of the control limbs (those treated with immobilization). Swelling of the limb decreased only in the group that received twenty-four hours of passive motion. Muscle mass increased by an average of 13 per cent (range, 4 to 34 per cent), in comparison with that of the immobilized limbs in every group that was treated with passive motion. Bone density was maintained only in the limbs in which the ankle became stiff (ankles that had been treated with passive motion for twelve hours or less). An inverse relationship was noted between the duration of passive motion and the radiographic density of the distal tibial metaphysis; this relationship was statistically significant (p < 0.01). The limbs treated for twelve, eight, or four hours each day showed progressively greater bone density in comparison with those treated with immobilization or with sixteen or twenty-four hours of passive motion.