The proposed studies are addressed to the trophic maintenance of synaptic transmission at spinal motoneurons by sensory fibers arising from the muscle. When impulse conduction along the sensory nerve is chronically blocked with tetrodotoxin, the efficiency of synaptic transmission at motoneurons from the disused sensory fibers was found to be significantly enhanced. In contrast, section of the sensory nerve caused central synaptic depression. Recovery of synaptic function following restoration of peripheral sensory connections did not require muscle activity nor activity of the injured sensory fibers. It is concluded that the normal functions of motoneuron synapses rely on two distinct factors: (1) the axonal continuity of the sensory fibers, interruption of which causes central synaptic depression, and (2) the presence of sensory impulse activity, elimination of which results in synaptic enhancement. It is suggested that the recovery of central synaptic function during peripheral nerve regeneration is triggered by mechanical, rather than functional, contact of the injured sensory fibers with some structures in the muscle.