An approximation of the membrane resistance - input resistance - was measured with double-barrelled micropipettes in hindlimb motoneurons during wakefulness and the states of sleep in chronically implanted, restrained cats. During postural atonia in rapid-eye-movement (REM) sleep, motoneurons had a sustained hyperpolarization of 5-8 mV and a 30% decrease in input resistance. There was little difference in membrane potential or input resistance between wakefulness and the other sleep states. The results directly support the hypothesis that motoneuron deactivation in REM sleep arises from active inhibition.