1. In order to investigate the characteristics of synapses formed by a foreign nerve in skeletal muscle of the frog, the hypoglossal nerve in Rana pipiens was implanted under the cutaneus pectoris (c.p.) muscle which was denervated 1 month later.2. Within 1 week of crushing the c.p. nerve, hypoglossal nerve fibres could be seen extending towards the denervated end-plates and the muscle contracted in response to stimulation of the implanted nerve.3. The synapses formed by the foreign nerve differed from those of the original nerve in several ways. The mean quantal content (m) of end-plate potentials (e.p.p.s) evoked by stimulation of the hypoglossal nerve in c.p. was lower than that of e.p.p.s evoked by stimulation of the regenerated original nerve. The mean latency between stimulation of the implanted nerve and the onset of e.p.p.s was longer than that for stimulation of the original nerve. Stimulation of the foreign nerve often evoked multiphasic e.p.p.s whereas these were only seen on original nerve stimulation during the first 3 months after regeneration. These abnormalities of the foreign innervation persisted for at least 6 months after crushing the original nerve.4. The geniohyoid muscle (which is normally innervated by the hypoglossal nerve) and c.p. appear to contain similar types of muscle fibre. Furthermore, when the geniohyoid muscle became reinnervated by the hypoglossal nerve after it had been crushed, the synapses formed were more effective than those formed by the hypoglossal nerve in c.p.5. Even if reinnervation of c.p. by its nerve was delayed by cutting the brachial nerve at its exit from the vertebral column, the synapses formed by the hypoglossal nerve in c.p. remained abnormal, suggesting that an incompatability exists between the c.p. muscle and the hypoglossal nerve.6. In spite of the differences found between the synapses formed by the hypoglossal nerve and the original nerve in c.p., there was no evidence of regression of the hypoglossal innervation during the period of observation.