Neurons in the median preoptic nucleus (MnPN) and the ventrolateral preoptic area (vlPOA) express immunoreactivity for c-Fos protein following sustained sleep, and display elevated discharge rates during both nonREM and REM sleep compared to waking. We evaluated the hypothesis that MnPN and vlPOA sleep-active neurons are GABAergic by combining staining for c-Fos protein with staining for glutamic acid decarboxylase (GAD). In a group of 6 rats exhibiting spontaneous total sleep times averaging 82.2 ± 5.1% of the 2 hours immediately prior to sacrifice, >75% of MnPN neurons that were Fos-immunoreactive (IR) were also GAD-IR. Similar results were obtained in the vlPOA. In a group of 11 rats exhibiting spontaneous sleep times ranging from 20-92%, the number of Fos+GAD-IR neurons in MnPN and vlPOA was positively correlated with total sleep time. Compared to control animals, Fos+GAD-IR cell counts in the MnPN were significantly elevated in rats that were sleep deprived for 24 hours and permitted 2 hours of recovery sleep prior to sacrifice. These findings demonstrate that a majority of MnPN and vlPOA neurons that express Fos-IR during sustained spontaneous sleep are GABAergic. They also demonstrate that sleep deprivation is associated with increased activation of GABAergic neurons in the MnPN and vlPOA.