To investigate the regenerating ability and healing process of the rat Achilles tendon, the tendon was cut and its repair process was observed up to 6 months without hindlimb casting. The gap between severed tendons was filled with soft connective tissue at 3 days postoperatively and bridged by fibrous connective tissue at 1 week. By light microscopy, tendons showed a small number of tenoblasts with an ovoid nucleus and rich cytoplasm among tenocytes which contained a thin nucleus and scanty cytoplasm at 3 days. Tenoblasts increased in number at 1 and 2 weeks, and decreased at 4 weeks onward. Blood vessels were located at the peripheries of the regenerating portion at 3 days, increased in number and distribution throughout the regenerating portion and tendon at 2 weeks, and decreased at 4 weeks afterwards. Fibroblasts and collagen fibers at the regenerating portion were small in number at 3 days, increased at 1 and 2 weeks, and oriented along the long axis of tendon at 2 and 4 weeks. It was difficult to discriminate between severed tendon and the regenerated portion at 4 weeks. Cartilage was observed in some specimens at 4 weeks, and bones were found in all specimens at 3 and 6 months. These findings indicate that fibroblasts derived from both tendon and surrounding connective tissues are involved in repair, and that the repair process of tendon seems most active at 2 weeks after surgery. Regenerated tendons are considered to restore principal tendon structure at 4 or, at the latest, 6 weeks. Thus, Achilles tendon has an intrinsic ability to repair within several weeks, which provides a rationale for consideration of medical treatment and planning of early rehabilitation.