Fibres of the sterno-cleido-mastoid (s.c.m.) muscle normally innervated by the accessory nerve have been reinnervated by afferent fibres of the vagus nerve after supranodose vagal-accessory nerve anastomoses or direct implantation of the vagus nerve into the s.c.m. in cats, rabbits and sheep. The afferent fibres contributing to this reinnervation were confirmed to be cholinergic as transmission was blocked by gallamine, and histochemical evidence obtained of cholinergic motor end-plates. The association of the axons of cells of the nodose ganglion and s.c.m. muscle fibres was further demonstrated when horseradish peroxidase injected into the s.c.m. was detected in somata of nodose ganglia cells. The largest number of reinnervated motor units, fifty, identified by electromyographic recording from the s.c.m. muscle, represents a small proportion of the afferent fibres in the vagus. Factors contributing to this degree of reinnervation are discussed. Conduction velocities in afferent fibres involved in the reinnervation were in the range less than 2.5 to greater than 35 m/s with 36% being 6-12 m/s. Between 40 and 98.5% of the myelinated fibres of the accessory nerve replaced by vagal afferent fibres were less than 6 microns in diameter. The afferent nerve fibres involved in the reinnervation were associated with larynx, respiratory tract, oesophagus and stomach. Afferent discharges recorded as bursts of electromyographic potentials in the s.c.m. occurred during spontaneous movements of these structures and on their mechanical stimulation.