Introduction: The distraction osteogenesis (DO) is the biological process of new bone formation between segments separated by traction. McCarthy (1992) firstly applied DO for mandibular lengthening of patients with facial deformities. There are few experimental studies on the effect of DO rate on the quality and quantity of new bone. Objectives: To evaluate histologically and histomorphometrically the effects in new bone of two rates of canine mandible tooth-borne DO. Materials and Methods: 7 Beagle dogs weighing between 15-18kg were submitted to DO and 3 remained as a control group. Both hemi-mandibles were used for experimental purposes: Group A: 6 mandibles - control group, Group B: 7 hemi-mandibles - two daily activations of 0.5 mm; Group C: 7 hemimandibles - 1 mm single daily activation. The distraction period was followed by 12 weeks of bone consolidation. Histological and histomorphometric evaluation of new bone tissue was performed. Results: Centripetal bone tissue formation from the marginal host segments of distraction gap occurred in both groups B and C. In group B, the distraction gap was consistently bridged by a remarkable amount of new trabeculae bone. In group C, non-union situations and large areas of cartilage / fibrocartilage were frequent. Histomorphometric evaluation showed statistically significant differences in the coefficient of variation of bone formation between the medial-buccal and lingual-buccal areas. Conclusion: Increasing the rate of distraction produces structural changes in the newly formed bone. Clinical Implications: This research allows inferring that continuous DO may be effective for the correction of mandibular deficiencies. Schlagwörter: Distraction osteogenesis, endochondral ossification, intramembranous ossification, tooth-borne distractor, histomorphometry, mandible, osteotomy