The study comprised 12 groups of female rats:​ 6 groups of intact rats killed at 2, 6, 9, 12, 15, and 24 months of age, 4 groups of rats ovariectomized at 6 months and killed together with the intact rats at 9, 12, 15, and 24 months of age, and 2 groups of rats (one intact and one ovariectomized) treated with estrogen (2 μg estradiol valerate/rat/week s.c.) for 8 months before they were killed at 24 months of age. The composition, dimensions, and mechanical strength of intact bone and bone collagen from femoral diaphyses were investigated in relation to age, ovariectomy, and estrogen administration. Up to 6–9 months of age, the axial length, percentage ash, density, and compressive mechanical stress increased, whereas percentage collagen decreased. An age-related increase in bone mass, crosssectional area, and wall thickness and a decrease in mechanical quality of bone collagen were apparent from 2 to 24 months of age. An age-related periosteal bone formation and the absence of endosteal bone resorption were demonstrated in intact rats. Compared with intact rats, ovariectomy was followed by an increase in body weight, a tendency to reduced percentage ash and a depressed bone mass, crosssectional area, and wall thickness of femoral diaphyses. The compressive mechanical stress of intact bone and the mechanical quality of bone collagen were unaffected by ovariectomy. Ovariectomy did not influence the periosteal bone formation but induced an endosteal bone resorption not present in the intact rats. The estrogen treatment of the ovariectomized rats normalized the body weight of the rats and brought to an end the endosteal bone resorption induced by ovariectomy. Estrogen treatment of both ovariectomized and intact rats tended to reduced the rate of periosteal bone formation.