The effect of depriving the tibia of normal mechanical function during its growing period was investigated by performing a unilateral sciatic neurectomy in the gluteal region of young rats. The animals were kept for 16 months, after which their tibias were examined. On the neurectomized side there was a failure of this bone to attain its normal weight, thickness, cross sectional shape or longitudinal curvature. Attainment of the bone's overall length was uneffected.