We assessed the effectiveness of end-to-side nerve coaptation by evaluating the extent of reinnervation and the integrity of the donor nerve in the rat. Fifty-six Sprague-Dawley rats were randomly divided into "fresh" and "predegenerated" (delayed) repair groups with or without a window in the perineurium. The right peroneal nerve was cut and sutured to the tibial nerve in an end-to-side fashion. Two and 3 months postoperatively, we measured the nerve conduction velocity (NCV) of the peroneal and tibial nerves, took dry muscle weights, and performed histologic studies. Peroneal NCV recovered up to 71% (2 mo) and 79% (3 mo), and tibial NCV recovered up to 88% (2 mo) and 91% (3 mo). The latter results indicate that this procedure had a slightly negative effect on donor tibial NCV. Dry weight of the tibialis anterior muscle returned to 60% (2 mo) and 82% (3 mo) of that of the contralateral side. Histologic results showed that there were numerous regenerating axons in the distal end of the peroneal nerve. Presence of a window in the perineurium improved the histologic profile significantly.