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Previously, we have reported that nitric oxide synthases (NOSs), which generate NO, modulate fracture healing. However, the cellular sources of the NOS isoforms during the course of fracture healing have not been studied systematically. The purpose of this study was to localize the cellular distribution of NOS isoforms (inducible NOS [iNOS], endothelial NOS [eNOS], and neuronal NOS [bNOS]) by in situ hybridization and immunohistology after femoral fractures in rats. The iNOS signal was detected during the initial stages (on day 4 and day 7) of fracture healing in 52 ± 2% (mean ± SE, n = 7) of cells within the intramembranous region, along the edge of the periosteal callus. The iNOS signal in callus cells declined to an undetectable level on day 14. eNOS was detected during the middle stages (on day 7 and day 14) of fracture healing in cells lining the blood vessels and also in 49 ± 3% of cells in the chondral region. The bNOS signal was found to be increased at the later stages (day 14 and day 21) of fracture healing in 51 ± 3% of cells at the junction between fibrous tissue and cartilage within the fibrochondral region. In summary, the expression of NOS isoforms during fracture healing was time dependent and cellular distinctive.