In unrestrained, unanaesthetized cats the response of the motoneurones to direct electrical stimulation was recorded from muscular nerves of the hindlimb, following deafferentation of the animal and chronic implantation of a stimulating microelectrode in the ventral horn. No significant change in amplitude of this response was recorded in either extensor or flexor nerves during transition from quiet wakefulness to synchronized sleep. However a marked, tonic depression of the direct motoneuronal response, attributed to hyperpolarization of the motoneurones, occurred throughout the desynchronized sleep. Generally no additional change of the direct motoneuronal response occurred during the bursts of REM. The absence of phasic changes in motoneuronal excitability to direct stimulation during the bursts of REM suggests that the phasic depression of the monosynaptic reflexes occurring during the REM periods of desynchronized sleep is the result of presynaptic inhibition.