HAIR cells of inner ear sense organs and of lateral line organs in fish are innervated by afferent nerve fibres, to which they transmit information about mechanical stimuli. Efferent inhibitory nerve fibres, which originate in the central nervous system, also innervate most hair cells at their base (Fig. 1b). Recent evidence indicates that both these synapses are chemical; excitatory postsynaptic potentials (e.p.s.p.s) have been recorded in afferent nerve terminals in response to mechanical stimulation1,2 and inhibitory postsynaptic potentials (i.p.s.p.s) have been recorded in hair cells on stimulation of the efferent fibres2. Furthermore, several findings suggest that acetylcholine is the efferent transmitter: i.p.s.p.s in hair cells, as well as the ensuing inhibition of afferent discharge, can be blocked by curare2-5 both acetylcholine esterase and choline acetyltransferase have been found in the efferent fibres6-9. The nature of the afferent transmitter however has not been elucidated, although catecholamines10 and glutamate11 have been implicated.