Intercalary intrasynovial and extrasynovial flexor tendon graft donors were placed within the synovial sheaths of the medial and lateral forepaw digits of 22 dogs and treated with early controlled passive mobilization. Specimens were studied by light and transmission electron microscopy at 10 and 21 days. Early repair in the extrasynovial tendons occurred by an ingrowth of connective tissue from the digital sheath. Adhesions obliterated the gliding surface and occupied the space between the tendon's gliding surface and surrounding tissues. There was no epitenon response noted in the extrasynovial tendon grafts. While there was considerable new collagen fibril formation within the repair site at the ultrastructural level, there was a lack of longitudinal remodeling. In contrast, the intrasynovial tendon grafts showed early healing, with minimal adhesion formation, by a proliferation and migration of cells from the epitenon. These cells showed greater cellular activity and collagen production at 10 and 21 days compared to cells in extrasynovial tendons at the same intervals. The findings of this study suggest that the use of intrasynovial autogenous tendon graft donors, coupled with early controlled motion, stimulates an intrinsic repair process in both the tendon stump and autogeneous tendon graft. These findings differ significantly from the experimental findings in which extrasynovial, paratenon-covered grafts are used.