The effect of fracture of the tibia on the subsequent overgrowth of the femur in the injured limb has been studied in ad lib, -fed rats. The thrust of these studies was to determine if the time in the body rhythm (diurnal) when injury was sustained could influence the rate of postfracture linear bone growth. Accordingly, different groups of young rats were subjected to tibial fracture at one of 4 times in a 24 hour day (6 hours apart), and a ratio of femur lengths (injured left side/intact right side) in each group was calculated 30 days thereafter. The data indicated that midday was the only time when tibial fracture elicited significant femoral overgrowth. At this time, the femur length differential in the fractured rats was small (0.52 mm), but it was significantly greater than that observed in intact control rats. The diurnal response was not present in fractured hypophysectomized rats, nor could it be detected in normal rats by measuring the thicknesses of the growth cartilages from the intact and fractured tibias. However, in 2 separate studies, the patterns of radiostrontium retention in the tibias and in the femurs confirmed that midday was the time in the rat body rhythm when processes of ossification in the epiphyses and in the remodeling fracture callus proceeded most actively. The 24-hour retention of 85Sr in the fractured tibias was greater than that in the intact tibias (5 days postfracture), but the 5-day retention of isotope in the femurs on the side of tibial fracture (30 day postfracture) was significantly less than that of the bone in the uninjured limb. The skeletal responses to fracture are not limited to the injured element; the magnitude of the responses both in injured and intact bones of a limb have a diurnal component.