The aim of this study was to assess the effectiveness of reinnervation using end-to-side neurorrhaphy in the upper extremity of the rabbit. The cut right ulnar nerve was repaired and sutured to the side of the median nerve about 3 cm above the elbow joint. The extent of reinnervation was quantitatively evaluated, as well as the integrity of the intact donor nerve in 36 rabbits randomly treated with fresh or delayed nerve repair with or without perineurotomy. Evaluations included nerve conduction velocity (NCV) of both the ulnar and medial nerves, dry muscle weight, and histologic examination (neurofilament stain and morphometric assessment) at 3 and 6 months postoperatively. NCV recovery rates were 79% and 87% for the ulnar nerve, and 89% and 94% for the median nerve compared to contralateral intact nerves, at 3 and 6 months, respectively. Flexor carpi ulnaris muscle mass measurements revealed a recovery in dry muscle weight of about 81% and 88% at 3 and 6 months, respectively, compared to the intact contralateral flexor carpi ulnaris. Histologic studies with neurofilament staining reveal numerous axonal sprouts at the distal end of the median nerve, indicative of myelinated axonal regeneration. Morphometric analysis demonstrated no difference between fresh and delayed repairs. These results indicate that in the upper extremity of rabbits, end-to-side neurorrhaphy permits axonal regeneration from the intact donor nerve, and is associated with satisfactory recovery. The effect of the procedure on the donor nerve was negligible.