The present study was performed to generate data implicating glycine or gamma-aminobutyric acid as neurotransmitter candidates mediating the IPSPs which are recorded in lumbar motoneurons following electrical stimulation of the nucleus reticularis gigantocellularis (NRGc) during the atonia of active sleep. Accordingly, intracellular records were obtained from lumbar motoneurons in unanesthetized, normally respiring cats during naturally occurring states of active sleep, while inhibitory amino acid antagonists were microiontophoretically released next to the recorded cell. Electrical stimuli, applied to the NRGc during active sleep under drug-free conditions, evoked inhibitory postsynaptic potentials (IPSPs) in all of the lumbar motoneurons which were examined. These NRGc-induced IPSPs exhibited an average latency-to-onset of 26.6 +/- 1.3 ms, a latency-to-peak of 42.5 +/- 1.3 ms, an average amplitude of 3.9 +/- 0.4 mV and a duration of 34.4 +/- 2.1 ms. Strychnine, when applied microiontophoretically, abolished or markedly suppressed these NRGc-induced IPSPs. In contrast, the microiontophoretic application of picrotoxin or bicuculline methiodide failed to block these IPSPs. To the extent that strychnine may be considered to be a specific antagonist of glycine, the present results suggest that glycine (or a structurally related amino acid) participates in the generation of NRGc-induced IPSPs during the atonia of active sleep.