In the adult rat nervous system, motor neurons are recognized specifically by a monoclonal antibody, MO-1. Because binding by MO-1 is lost following axotomy, contact with the target may regulate this motor neuron-specific epitope. To test this hypothesis, we examined the recovery of MO-1 immunoreactivity in hypoglossal neurons following unilateral damage to the hypoglossal nerve. During the first week following nerve crush, neurons in the ipsilateral hypoglossal nucleus lost all immunoreactivity for MO-1. Antibody binding returned with time, and by 4 weeks, 80% of the injured neurons had recovered the MO-1 epitope. Since motor neurons reinnervate their original targets readily following nerve crush, it appears that MO-1 binding is recovered when motor neurons return to their original target muscles in the tongue. When the hypoglossal nerve was cut and inserted into a foreign muscle nearby (the sternomastoid muscle), the MO-1 epitope was not detected in the injured neurons, even when examined 6 weeks after surgery. However, if the sternomastoid muscle was denervated prior to insertion of the hypoglossal nerve, thus allowing the hypoglossal nerve to synapse with this foreign target, increasing numbers of hypoglossal neurons reacquired MO-1 immunoreactivity with time. Our results suggest that the MO-1 epitope is only expressed in motor neurons that are in synaptic contact with skeletal muscle. Thus, a property that distinguishes mature motor neurons from other neuronal phenotypes appears to be regulated by direct synaptic interaction with the postsynaptic target.