In order to investigate fibroblast survival and proliferation in autogenous flexor tendon grafts, hindlimb intrasynovial and extrasynovial donor tendons were placed within the synovial sheaths of the medial and lateral forepaw digits of 21 dogs (42 tendons) and treated with controlled early passive motion. Intravital histologic evaluations with confocal microscopy and biochemical determinations of total DNA content and DNA synthesis were carried out at 10 days, 3 weeks, and 6 weeks. Intravital staining of the extrasynovial tendon grafts demonstrated variable degrees of cellular necrosis at the earliest intervals followed by cellular repopulation with fibroblasts and neovascularization from surface vessels. In contrast, intrasynovial tendon grafts were populated predomnantly by viable cells at each interval, with occasional patches of cell necrosis and fibroblast ingrowth. Total DNA content and DNA synthesis values in the intrasynovial donor tendons were significantly lower than those seen in the extrasynovial tendon grafts at each interval. Extrasynovial tendons appear to act as scaffolds, undergoing extensive cellular death followed by a rapid repair response. Findings that intrasynovial tendon fibroblasts survive the tendon grafting process suggest that the nutritional supplies and metabolic requirements of intrasynovial and extrasynovial donor tendons differ largely.