Excessive tension across a nerve repair is known to impair nerve regeneration. However, it is uncertain whether nerve grafting is necessary when end-to-end repair would result in only mild to moderate tension. This study investigated the effect of tension on nerve regeneration. Sciatic nerves of 48 Lewis rats were transected and then repaired primarily after resection of 0-, 3-, 6-, or 9-mm lengths of nerve. Postoperative tension levels were quantified using a tensometer. Robust nerve regeneration was observed at 4 weeks in all except the 9-mm repair group, which showed lower nerve fiber counts, percent neural tissue, and nerve density (P < 0.05) and decreased functional recovery. These data indicate that modest levels of tension are well tolerated, but nerve regeneration drops precipitously once a critical tension threshold is exceeded. This threshold was between 0.39 and 0.56 N in the model studied, corresponding to a nerve defect between 6 mm and 9 mm.