1. The factors leading to long-bone overgrowth after fracture of the tibia in the growing rat are partly regional in effect and produce overgrowth in the femur of the same limb.
2. As evaluated by the plastic injection technique, vascular-bed hypertrophy after fracture does not appear to be related to the overgrowth phenomenon in the rat. On the other hand, it is possible that arterial-flow changes after fracture may play a part in the production of the growth alterations demonstrated in this experiment.
3. Immediately following fracture of one tibia in the immature rat, femoral growth is temporarily retarded on both fractured and non-fractured sides. This is followed by a period in which both fractured and non-fractured limbs show overgrowth. This femoral overgrowth is significantly greater in the fractured limb than in the non-fractured limb. Growth retardation of the unfractured side did not seem to contribute significantly to the observed differences.