Although evidence suggests that the basal forebrain contains a hypnogenic mechanism, putative sleep-promoting neural elements within this area have not been identified. We examined basal forebrain neuronal activity during waking, non-rapid-eye-movement (NREM) sleep, REM sleep and various transition states. Based on state-related discharge rates. 3 cell types were defined. Thirty-nine of 83 cells were classified as waking-active, i.e. waking discharge rates were greater than 2 times NREM sleep rates. Twenty-three of 82 cells were classified as state-indifferent (waking and NREM rates differed by a factor of less than 2). NREM sleep discharge rates of the remaining 20 cells were greater than 2 times waking rates. These were labeled sleep-active cells. Discharge rates of these cells during epochs of alert waking were low, averaging less than 1 spike/s. Maximal discharge rates occurred during NREM sleep, averaging 9.44 spikes/s. Increased discharge of sleep-active cells anticipated sleep onset; cells had an average discharge rate of 6.60 spikes/s during transitions between waking and NREM sleep. Sleep-active cells were confined to the ventral basal forebrain, in the horizontal limb of the diagonal bands of Broca, substantia innominata, entopeduncular nucleus and ventral globus pallidus. These areas overlap, in part, with those where chemical, thermal and electrical stimulations evoke sleep, and where lesions suppress sleep. Based on location and discharge pattern we consider sleep-active cells candidates for mediating some of the sleep-promoting functions of the basal forebrain.