Introduction: Nerve regeneration through end-to-side coaptation has been confirmed and these fibers have been believed often to sprout from inter-nodal space of the recipient nerve. We used recombinant adenovirus vector containing LacZ gene for the tracing of the peripheral nerve fibers and showed that the regenerating nerve fibers can be traced along its length. In the previous study using peroneal nerve coapted to the tibial nerve in end to side fashion, the regenerating fibers sprouted from very proximal to the original neuron of the peroneal nerve and there was no discernible inter-nodal sprouting from the recipient nerve. Experimental model: Eighty-four rats were divided into two experimental groups. In the experiment I, root levels of the normal musculocutaneous, median and ulnar nerve were detected using 27 rats. Four weeks after the infection of the adenovirus to these three nerves. In the experiment II, the severed proximal musculocutaneous nerve was connected to either median or ulna nerve through perineurial window in ETS manner at the first operation. Second operation for the adenovirus infection was carried out 6 weeks after the first operation. Four weeks after the second operation, the animals were euthananized for the ß-galactosidase chemistry. RESULTS: Positive tracing ratios: Twelve rats demonstrated positive staining of the infected nerve (12/27 44.4% positive ratio). In experiment II, median nerve model showed 50 % positive ratio (15/30) and the ulnar nerve model 66.7% (18/27) Distribution of traced nerve fibers: Experiment I. Traced nerve fibers in the brachial plexus with infection to the musculocutaneous nerve distributed C5 to C7 root level. Those in the median nerve control ranged in C6 to Th1 roots. Ulnar nerve control demonstrated traced nerve fibers in the C8 and Th1. Experiment II. In the median nerve recipient group, most of the regenerating fibers were coming down from far proximally. Ulnar nerve recipient group demonstrated inter-nodal sprouting at near to the end-to-side coaptation site if there was no demonstrable pathway from upper brachial plexus roots to the lower roots. Mixture of the regenerating fibers from the proximal upper brachial plexus roots and inter-nodal space at near to coaptation site were demonstrated in the group with demonstrable pathway from upper to lower roots. Discussion:: This study suggests novel fact that neurotropism to attain more appropriate reinnervation such as sprouting from the original neurons of the injured nerve may work in end-to-side nerve repair.