The present study examined the effects of strychnine on the tonic hyperpolarization and the changes in membrane properties of lumbar motoneurons that occur during active sleep. To carry out these studies, intracellular recordings from lumbar motoneurons were combined with the juxtacellular microiontophoretic application of strychnine in chronic, undrugged, normally respiring cats. During active sleep, compared to quiet sleep, motoneurons that were not exposed to strychnine exhibited tonic hyperpolarization, a decrease in cell excitability, and an increase in membrane conductance; they were also bombarded by high- frequency, large-amplitude IPSPs. In conjunction with the juxtacellular application of strychnine, there was a marked reduction in the degree of hyperpolarization during active sleep; motoneuron excitability was no longer suppressed, and there was a reduction in the increase in membrane conductance. In addition, the large-amplitude IPSPs were blocked. These results identify glycine as the neurotransmitter responsible for the state-dependent changes in membrane properties and the hyperpolarization of motoneurons that takes place during active sleep.