The response of healing canine flexor tendons to motion was investigated using protected passive mobilization techniques. Early motion, delayed motion, and immobilization groups were compared over a 12-week period for their strength and excursion characteristics. Tendons which were mobilized early showed progressively greater ultimate load and linear slope values at each time interval tested. The ultimate load of the immediately mobilized tendons, those tested at 3 weeks, was twice as great, and the linear slope values were almost three times greater than the immobilized repairs at 3 weeks. Similar differences were noted at each time interval through 12 weeks. The differences in angular rotation of the distal interphalangeal joint following the application of a small load were also significant. At 12 weeks, the angular rotation of the tendons of the immobilization group averaged only 19% +/- 2% of their intact contralateral controls. The delayed mobilization tendons produced values of 67% +/- 8%, and the immediate mobilization tendons produced 95% +/- 10% of the control joint motion. These findings indicate that early protected passive mobilization augments the physiologic processes that determine the strength and excursion of repaired flexor tendons.