The basal forebrain including the preoptic area and anterior hypothalamus plays an important role in regulation of slow wave sleep. To examine whether this area is concerned also in a neural mechanism of paradoxical sleep, single neuronal activity in and around the preoptic area along with cortical EEG and neck EMG was recorded in undrugged, 12-h sleep-deprived rats whose head was fixed painlessly. The neurons whose activity was recorded during all states of sleep and wakefulness (n = 98) were classified into following 5 groups according to their firing behavior in relation to sleep-waking states. (1) Neurons most active during paradoxical sleep (n = 26). (2) Those most inactive during paradoxical sleep, some of which were inactive also during slow wave sleep (n = 16). (3) Those specifically active during slow wave sleep (n = 14). (4) Those less active during slow wave sleep than during wakefulness and paradoxical sleep (n = 9). (5) Those whose activity showed no clear correlation with the sleep-waking states (n = 33). About one third of neurons in each of groups (1) to (4) began to increase their sleep-related activity in advance of the shift of sleep/wakefulness state recognized in EEG. These results suggest that the preoptic and anterior hypothalamic areas are involved, at least in rats, in regulation of not only slow wave sleep but also paradoxical sleep.