The functioning modalities of the efferent system were analysed in the isolated frog labyrinth. The efferent synapses of the posterior canal were activated via an axon reflex by antidromic electrical shocks (10-200 Hz) applied for increasing times (250 ms-10 s) to the anterior-horizontal nerves. Either decrease (inhibition) or increase (facilitation) in the resting discharge rate were observed in the majority of the units examined. Inhibition and facilitation, however, are peculiar to any given unit since inhibition does not reverse to facilitation or vice-versa. This fact as well as the long response latency (not less than 10 ms) and the linear dependence of both effects on the stimulation frequency suggest that inhibition and facilitation are due to the repetitive activation of two different types of efferent fibres synapsing on the hair cells. The drastic modifications in the afferent synaptic discharge produced by full activation of the efferent system indicate that the static properties (response asymmetry) as well as the dynamic properties (response adaptation) of the mechanically driven afferent response can be substantially controlled by the central nervous system at the receptor level.