The purpose of this study was to evaluate the effects of the mandibular retractive force on the mandibular condyle of growing dog. The experimental animals were six mongrel dogs of two-month old. Their deciduous dentition were completed. Two of them was used as control group, and experimental group was composed of remaining four. Head band and chin cup were made of cotton tape, and hooks are fabricated on the chin cup and had band for closed coil. Mandibular retractive force was 100g/side and chin cap appliance was used for 14 hours/day during night. Experimental group were sacrificed at 2, 4, 6, 8 weeks from beginning of the experiment. Right TMJ was prepared for histologic study and left TMJ was examined grossly for disc, fossa, and condyle. The conclusions are: 1. Two-month old control animal showed active cartilaginous growth on the mandibular condyle, therefore showed thick proliferative and hypertrophic zones. Remodeling process in the condyle head was observed in which there were bone resorption on the anterior surface and bone apposition on the posterior surface. 2. Four-month old control animal showed marked reduction of hypertrophic zone but the condylar bone remodeling was more pronounced. 3. In experimental group, there are marked reduction of hypertrophic zone at 4 weeks from beginning of experiment, and hypertrophic zone disappeared at posterior-superior portion of condyle in 6-week experimental animal. 8 week experimental animal showed slight recovery of hypertrophic zone. 4. In experimental group, bone deposition was increased at anterior surface of condyle, and bone resorption was increased at posterior surface of condyle. 5. In control group, the glenoid fossa and surrounding bone showed mainly bone apposition. But experimental group showed bone resorption at anterior surface of articular eminence and increased bone apposition at posterior surface of postglenoid spine. 6. No marked traumatic change was seen but 4 weeks and 8 weeks experimental animal showed flattening of posterior surperior condylar surface. Bone marrow of condyle showed minute focal bleeding in 2 weeks and 4 weeks experimental animal, and congestion and depression of hematopoietic bone marrow during all experimental period.