The torsional behaviour of the femur has been studied in 145 male Wistar rats, aged 3-36 weeks. The ultimate torsional moment and the torsional stiffness of the femora increased with increasing animal age. In contrast, the femoral deformation at failure, the ultimate torsional angle, was almost constant for all age levels. The strength and stiffness of cortical bone as a material, the ultimate torsional stress and the modulus of rigidity reached a plateau when the rats were about 14 weeks old, and had a body weight and femoral length of about 350 g and 35 mm, respectively. In conclusion, cortical bone of Wistar rats probably reaches maturity at an age of about 14 weeks.