This study was undertaken to assess the effects of demethylchortetracycline of bone growth of mandibular symphyseal region in rats. Demethylchlortetracycline at 30mg/kg body weight, respectively, were daily administered by mouth to the seven female rats from 10th day pregnancy to 13th day. Thirty six new-born rats from these experimental animals were used for histological examination at 1, 5, 10, 15, 20, 25 and 30 days. All these new-bone rats were killed by an overdose of ether. Mandivular bodies were removed and fixed in 10% neutral formalin, Carnoy and aceton. Specimens were embedded, sectioned and stained with H-E, Van Gieson, PAS and prepared for alkaline phosphatase. The results were as follows; 1. Until erupting of incisors, hyaline cartilage was located in relatively large symphyseal space, but bone trabeculae of ossifying area at incisal were arranged irregularly in experimental group. 2. During this period, PAS reaction was moderately positie, but alkaline phosphatase reaction was slightly positive. 3. By erupting or incisors, symphyseal space appeared narrower like control group, but alkaline phosphatase reaction tended to slow down. 4. By erupting of molars, symphyseal space appeared much narrower, and cartilage plate was reduced and sealed off like control group. Alkaline phosphatase reaction tended to slow down severely.