Temporal and spatial expression of bmp-2 and bmp-4 was investigated in distraction osteogenesis and fracture healing models in order to delineate their roles in these new hone formation processes. Distraction osteogenesis was performed on the left tihia diaphyses of Sprague-Dawley rats, and was confirmed by serial radiographs and histologicaJ examination. Total RNA was isolated from the distraction gaps and fracture sites sequentially until the postoperative eighth week, and mRNA expression was quantitated hy competitive reverse transcription-polymerase chain reaction(RT-PCR) using specific primers for bmp-2 and bmp-4. In-situ hybridization was performed on the undemineralized tissue section of distraction osteogenesis group. Both bum-2 and bmp-4 mRNA expression increased during distraction osteogenesis and fracture healing. Increase in bmp-2 mRNA expression was more marked and prolonged in distraction osteogenesis compared with fracture healing, whereas bmp-4 mRNA expression was persistently increased in both groups. In-situ hybridization study revealed that hoth bmp-2 and bmp-4 were expressed at eariy osteohlasts producing osteoid, trabeculae-lining osteoblasts and osteocytes of regenerate bone. Enhanced new bone formation in distraction osteogenesis is associated with prolonged and increased expression of BMPs, especially BMP-2. lnspite of their structural similarity, BMP-2 and BMP-4 appear to have their distinct roles in new hone formation.