The purpose of this study was to investigate kinetics of the osteblast cell lineage in the periosteum and endosteum according to different distraction rates in callotasis of rats' Tibiae. 120 rats underwent osteotomy at the proximal metaphysio-diaphyseal junction of the left tibia for callotasis. Lengthening was started with varying distraction rates of 0.25 mm (group I), 0.5 mm (group II), 0.75 mm (group III), 1.0 mm (group IV) until 3.5 mm length gain was achieved. The animals that had osteotomy alone without lengthening served as a control(group V). Immunohistochemical staining of proliferating cell nuclear antigen(PCNA), osteocalcin and transglutaminase C(TGase C) were done on the four animals on each group sacrified at post-distraction 1, 3, 5, 7, 14, 28 days in order to observe the temporal changes among the experimental and control groups. At each examination, radiographic and histological studies were also done in order to correlate the immunohistochemical findings. The results obtained are summarized as follow; 1. The staining rate of PCNA was highest at the early distraction(day 1) phase and subsequently decreased in all groups. The staining rate of the cells in the periosteum was significantly higher than that of the cells in the endosteum (p<0.01). 2. The expression rates of osteocalcin in the periosteum of all groups were significantly higer than those in the endosteum (p<0.01). 3. The expression rates of TGase C in the periosteum of all groups were significantly higer than those in the endosteum (p<0.05). 4. Radiological and histological studies revealed that successful regenerate bone healing was achieved in groups, I, II and III but not complete in group IV. In conclusion, immunohistochemical study on callotasis of rats' tibiae revealed that the osteoblast cell lineage in the periosteum is more activated than that in the endosteum for proliferation and differentiation by distraction, suggesting that the periosteum plays a more important role in neo-osteo-genesis in the distraction gap. Daily distraction rate range of 0.25 mm to 0.75 mm in two increments is the appropriate for successful distraction osteogenisis of rat's tibia, but the rate of 0.25 mm a day is significantly better than that of 0.75 mm upon immunohistochemical observation.