The effect on bone growth after stripping of the periosteum of the middle third of the femur was investigated in 70 rats. Seventy further animals were used as controls. The animals were serially killed at 1, 2, 4, 8, 12, 16 and 20 weeks after operation. The femoral length, intertrochanteric diameter, diameter of the distal epiphysis of the femur, diameter of the femoral diaphysis and the tibial length were measured. Significant lengthening of the femur was noted only after 4 weeks, when it was 1.5% longer than the opposite side (p 0.001), but this was not maintained. The diameter of the distal epiphysis and the tibial length were not significantly affected. The intertrochanteric diameter was significantly shorter on the stripped side at the 2nd week and significantly longer at the 12th. The diaphyseal diameter of the stripped bones was progressively more narrow than the contralateral from the 2nd week. Differences remained significant at all stages of this study. Periosteal stripping yields only a temporary slight increase in bone length with a definite decrease in diaphyseal diameter.