The biomechanical effect of functional loading was studied in surgically tenotomized, repaired and immobilized right Achilles tendons of 34 rabbits. Beginning from the 5th day after surgery, loading was initiated by removing the immobilization casts of the animals to permit unrestricted weight-bearing to tolerance. At each of 12, 18 and 21 days after surgery, functionally loaded tendons and their corresponding nonloaded controls were excised and compared for differences in tensile strength, tensile stress and energy absorption capacity. Functional loading induced a twofold increase in the tensile strength (P less than 0.05) and energy absorption capacity (P less than 0.05) of the tendons, at both 12 and 18 days after surgery. No statistically significant differences were observed in the tensile strength and energy absorption capacity of the tendons at 21 days after surgery. Statistically significant differences in tensile stress were not observed at any time period throughout the study. These findings demonstrate that functional loading augments the tensile strength and energy absorption capacity of experimentally tenotomized tendons without promoting re-rupture, but only during the very early stages of healing. Even though the healing process of rabbit tendons may differ from healing of human tendons, these results suggest that similar beneficial effects may be attained if repaired human Achilles tendon ruptures are carefully loaded during the very early rather than later stages of healing.