This response of condyle and femur was evaluated by histomorphometry. Five female Sprague-Dawley rats, 6 weeks of age, were subjected to bilateral ovariectomy, and five others were sham-operated. They were then killed 32 days later. Ovariectomy significantly increased the total thickness of the condylar cartilage and distal femoral growth-plate cartilage in the young, growing rats. Ovariectomy caused a 2-fold increase in thickness of the proliferative layer and a 4-fold increase in thickness of the hypertrophic layer in the condylar cartilage, and a 1.3-fold increase in thickness of the proliferative layer in the epiphyseal growth cartilage of the femur. Ovariectomy had no effect on the percentage of trabecular bone volume, the percentage of the bone surface covered by osteoblasts (Ob.S/BS) and osteoclasts (Oc.S/BS), and number of osteoclasts per bone surface (N.Oc/BS) in the mandibular condyle. On the contrary, ovariectomy caused a 68.5% decrease in bone volume, a 4-fold increase in Ob.S/BS, and 2-fold increases in Oc.S/BS and N.Oc/BS in the secondary spongiosa of the rat distal femur. Thus there was a prominent difference in the response to oestrogen deficiency between the mandibular condyle and femur in young growing rats.