1. Re-innervation of soleus was studied in the mouse after either crushing the sciatic nerve or re-implanting the nerve to soleus outside the original end-plate region.2. During the early stages of re-innervation subthreshold end-plate potentials (e.p.p.s) were recorded in muscle fibres in response to nerve stimulation. Later the e.p.p.s became large enough to evoke action potentials in muscle fibres.3. The rate of recovery of the release of acetylcholine (ACh) as estimated from the quantal content of e.p.p.s was faster when nerves re-innervated the old end-plate region after nerve crush than after re-implantation so that new neuromuscular junctions were formed.4. During re-innervation soleus fatigued more rapidly than normal during repetitive nerve stimulation. The fatigue was due to failure of neuromuscular transmission associated with an impaired release of ACh.5. During re-innervation soleus was supersensitive to ACh until nerve stimulation was capable of evoking action potentials in muscle fibres.