The effects of the calcitonin were determined on osteoporosis induced in the rat by ovariectomy. 90 rats were ovariectomized at the beginning of the study and 30 rats had sham-operation. Shamoperated 30 rats(group I) had no injection. In group II (30 rats), 0.5cc of normal saline was injected intramuscularly 4 times a week for 16 weeks. In group III (30 rats), 2.5MRC unit/kg of (Asu(1,7))-eel calcitonin was injected intramuscularly 4 times a week for 16 weeks after ovariectomy. All the rats were killed at the 16th week after the experiment. Sections of the femoral shaft were stained for Villaneuva bone stain and evaluated histomorphometrically. Ovariectomy produced decreased weight of the femur and significant increase in medullary area and periosteal bone formation were occurred. The increase in medullary area resulted from an increase in osteoclast number and size. In contrast, treatment of ovariectomized rats for 16 weeks with calcitonin prevented the increase in medullary area and the decrease of trabecular bone in ovariectomized rat. Also the activity of osteoclast was inhibited by calcionin treatment. The result indicated that, in the ovariectomized rat, calcitonin had an effect of inhibition of bone resorption but did not have a definite effect of new bone formation.