Hypnogenic structures are mainly located in the preoptic area of the hypothalamus. The number of Fos-immunoreactive neurons in the ventrolateral preoptic area (vlPOA) and in the median preoptic nucleus (MnPN) is positively correlated with the amount of preceding sleep. Majority of neurons recorded in these nuclei exhibit elevated discharge rates during sleep. Present study examined the effects of MnPN electrolytic lesion on the sleep-waking behavior in the rat. Experiments were performed on adult Mongrel albino male rats. Rats were implanted with EEG/EMG electrodes and with an electrode for MnPN lesion. Following a post-surgery recovery, rats were adapted to the recording environment, recorded for a baseline sleep-wakefulness cycle and then assigned to two groups. One group of rats was subjected to MnPN lesion and recorded for spontaneous sleep on the third, seventh and fourteenth days following the lesion procedure. Other rats served as a control group. The control and the MnPN-lesion rats exhibited different profiles of sleep-wakefulness organization. MnPN-lesion rats had significantly higher amounts of wakefulness and lower amounts of sleep, compared to control animals. Our findings support a view that the activity of a subset of MnPN neurons is strongly related to sleep regulation.