The effect of tension on tendon healing was evaluated in vitro using cyclic tension and no tension groups of chicken flexor digitorum profundus tendons and histologic and immunohistologic techniques. A Vitrodyne force-loading machine was used for application of cyclic tension on partially lacerated chicken flexor tendons in culture media. Laceration sites under cyclic tension after 14 days were covered by newly proliferated fibroblasts, aligned in the direction of tension. This new growth was much thicker than that seen in the no tension group at the same time interval. Procollagen synthetic activity began at 3 days of culture in both groups. At 21 days, newly formed fibroblasts in the cyclic tension group were stained positive more strongly at the surface layer than in the deeper layers. In the no tension group, the staining was primarily in the surface layer. Cyclic tension stimulated the intrinsic response of lacerated flexor tendons significantly more than no tension did by enhancing proliferation and migration of fibroblasts, as well as stimulating collagen synthesis.