We studied the effects of transforming growth factor-beta 1 (TGF-beta 1) on the genetic expression of procollagen type I and III and its effects on structural properties in the early stages of healing in rat Achilles tendon. The Achilles tendons in 90 rats were transsected and repaired immediately. TGF-beta 1 dissolved in phosphate-buffered saline was injected locally at the repair site using two different doses, and outcomes in both groups were compared to that in the control group given phosphate-buffered saline only. Five animals in each group were killed at one, two, and four weeks postoperatively, and the healing tendon was evaluated. A dose-dependent increase in the expression of procollagen type I and III mRNA was found one week postoperatively. The failure load and stiffness of the healing tendon were increased by TGF-beta 1 at two and four weeks.