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Considerable advances have been made in the knowledge regarding fracture healing and treatment duringrecent decades. Nevertheless, in 5-10 per cent of fractures, there are healing complications leading to complextreatment conditions, discomfort for patients and considerable costs. Therefore, there is a need for furtheradvances in the understanding of fracture healing. Several clinically relevant animal models have been usedto study the biological mechanisms of the bone tissue repair process during fracture healing. However, informationregarding reproducibility of such models is limited, which makes comparison of studies difficult.AIMS: To develop a reproducible and clinically relevant experimental fracture model in rat. To investigatehow fracture environment and fracture fixation stability influence the temporal distribution of ossificationin experimental fractures in this model. To investigate the possible influence of fracture environment andfracture fixation stability as well as the temporal distribution of ossification on biomechanical stability duringfracture healing.To study neovascularization during ossification in healing fractures.METHODS: A reliable and reproducible method for experimental external fixation of long bones in rats wasdeveloped.The ossification in external fixated femur osteotomies was studied with biomechanical, histological,histomorphometrical, immunohistochemical, microfil perfusion and transmission electron microscopy methods.RESULTS AND CONCLUSIONS: The method for experimental external fixation of long bones in rats wasfound to be reliable and reproducible for investigations of bone repair. In the early phase of fracture healing,periosteal ossification and intramedullary ossification were found to be histologically similar and were notinfluenced by fracture fixation rigidity or fracture environment. The intramedullary ossification was foundto be of considerable magnitude if tissues associated with the intramedullary region were preserved duringfracture fixation.Woven bone might be a prerequisite for the differentiation process of endochondral ossification.The magnitude of chondroid tissue is suggested to be of importance for the biomechnical stability inhealing fractures. Delayed fracture healing may possibly induce systemic stimulation of bone mass.There was considerable similarity between the neovascularization during endochondral ossification inepiphyseal growth plates and experimental fracture healing. The results strongly support the concept thatendochondral ossification during fracture healing is a recapitulation of endochondral ossification duringembryonic development of long bones.