A correlated intracellular and extracellular study of lumbar motoneuron excitability during sleep and wakefulness was performed in the chronic, unanesthetized, undrugged, normally respiring cat. Experiments were designed to reveal the extent to which hypotonia during active sleep in mammals is dependent on postsynaptic inhibition of somatic motoneurons. Variations in the antidromic field potential, antidromic and orthodromic spike, EPSP, membrane input resistance and rheobasic current were studied. No change in motoneuron excitability occurred when quiet wakefulness was compared to quiet sleep. A decrease in excitability was present, due to postsynaptic inhibition, during active sleep. Further phasic decreases in excitability, also due to postsynaptic inhibition, occurred during active sleep in conjunction with clusters of rapid eye movements.