1. The hemidiaphragm of the adult rabbit has a single band of end-plates running around the middle of the muscle. A study has been made of the formation of synapses during spontaneous reinnervation of this muscle, using histological, ultrastructural and electrophysiological techniques.2. Following spontaneous reinnervation, silver-stained nerve terminals were found in association with cholinesterase-stained end-plates only in the region of the muscle corresponding to the original innervation band.3. The regenerated nerve terminals were observed with the electronmicroscope in positions overlying or adjacent to the old synaptic folds.4. Spontaneous miniature end-plate potentials and evoked synaptic potentials were recorded only in the middle of the muscle fibres after reinnervation.5. The growth of the regenerating axons was not oriented towards the end-plate zone but followed muscle fibres and blood vessels in random directions.6. It is concluded that, in adult mammalian striated muscle, the old end-plate region is preferentially reinnervated as a consequence of some special property of the muscle fibre at this site.