While cholinergic stimulation of the PRF evokes a REM-like state, electrical stimulation of LDT/PPT neurons has not been used to test the hypothesis of mesopontine cholinergic control of REM sleep. Adult cats were implanted for electrographic recording and with bipolar unilateral stimulating electrodes, either in the LDT or within the PRF (stimulation control). Baseline recordings of the normal sleep-wake cycle were carried out for 5 h. On the next day, continuous stimulation of the LDT or mPRF was carried out during the same time period (0.5 ms pulses, 1 microA, 8 Hz) and with post-stimulation recording for 3 h. A second baseline recording day followed with same protocol as the first baseline day. This 3-day sequence, separated by 3 days, was repeated three times in each of the three LDT and the three medial PRF cats. Five hours of chronic low-amplitude stimulation of the LDT induced a highly significant increase in total REM and in the duration of REM sleep bouts. Stimulation of the mPRF did not affect any of the behavioral states. This study, the first to our knowledge to use low-amplitude stimulation of LDT in freely moving cats, indicates the importance of mesopontine cholinergic neurons in REM sleep.