In the chronic cat, using implanted electrodes, monosynaptic reflexes (MSR) of lumbar motor neurons were evoked by sciatic nerve stimulation and recorded from the tibial and peroneal nerves. The MSR activity was compared in 2 states of natural sleep: sleep with slow waves and spindle bursts in EEG [electroencephalogram] and sleep with low voltage fast waves in EEG, silence of neck EMG [electromyography] and appearance of rapid eye movements. It was found that in the latter sleep state the MSR size decreased and its threshold became higher. These depressive effects were abolished by complete spinal transection. These effects were observed in the innervation to both phasic and tonic components of extensor muscles which gave the 2 distinct peaks in the tibial nerve response.