Selectivity of synapse formation after nerve regeneration was tested in the parasympathetic cardiac ganglion of frogs (Rana pipiens). First, we tested the ability of somatic motor axons to establish synaptic connections with denervated ganglion cells by implanting the hypoglossus nerve into the vagotomized heart. After several weeks, stimulation of the implanted hypoglossus mediated a parasympathetic-like inhibition of the heart rate, and synaptic responses produced by hypoglossal stimulation were recorded intracellularly in ganglion cells. Light and electron microscopy indicated that implanted hypoglossal nerve terminals contacted parasympathetic ganglion cells only on their axons and not on the cell body (where most vagal synapses are found in control animals). Second, we tested whether regenerating vagal preganglionic axons would complete with foreign (hypoglossal) terminals for innervation of cardiac ganglion cells. We allowed the vagus nerve to regenerate in animals in which the implanted hypoglossus had established functional contacts with the cardiac ganglion. Vagal axons were able to reinnervate the heart and reestablish synaptic connections on the cell bodies of ganglion cells. Furthermore, functional transmission at the foriegn (hypoglossal) terminals disappeared concomitant with vagal reinnervation.