The influence of deficiency and administration of estrogen on the discrete phases of decalcified bone matrix induced endochondral bone formation was investigated. In ovariectomized (Ovx), sham operated (Sham) and sham operated and estrogen administrated (Sham+E) rats, chondrogenesis, cartilage degeneration and bone formation, and bone remodeling were occurred 7, 14 and 21 days after implantation of bone matrix, respectively. Calcium contents and alkaline phosphatase activity were less in Ovx than in the other groups during the experiment. Alkaline phosphatase activity was not different between Sham and Sham+E rats but calcium content was more on day 14 and less on day 21 in Sham+E than in Sham group. Calcium dependent ATPase activity was increased in Sham on day 21 but the activity was decreased in Sham+E animals. It is suggested that cartilage and bone differentiation were occurred notwithstanding estrogen deficiency and administration. Though estrogen deficiency suppresses calcification of bone and cartilage, exogenous estrogen induces rapid calcification of cartilage and suppresses calcium deposition of bone which may be related to calcium dependent ATPase activity.