Healing canine flexor tendons were treated with either total immobilization and were studied by light, scanning, and transmission electron microscopy at ten, twenty-one , and forty-two days. The immobilized tendons healed by ingrowth of connective tissue from the digital sheath and cellular proliferation of the endotenon. The ingrowth of reparative tissue from the digital sheath overwhelmed the epitenon response. At the ultrastructural level, collagen resorption was prominent whereas protein synthesis was limited. This was observed at all study-intervals. In contrast, the mobilized tendons healed by proliferation and migration of cells from the epitenon. Ingrowth of reparative tissue from the tendon sheath was notably lacking in this group. The epitenon cells exhibited greater cellular activity and collagen production at each interval compared with cells of the immobilized repairs.