The purpose of this study was to determine whether neurons in the medial pontine reticular formation with high discharge rates during REM sleep could be localized in regions of the brainstem having neurons displaying choline acetyltransferase immunoreactivity. Six cats were implanted with sleep recording electrodes and microwires to record extracellular potentials of neurons in the pontine reticular formation. Single-units with a S:N ratio greater than 2:1 were recorded for at least two REM sleep cycles. A total of 49 units was recorded from the pontine reticular formation at medial-lateral planes ranging from 0.8 to 3.7 mm. The greatest proportion of the units (28.6%) showed highest discharge during active waking and phasic REM sleep compared to quiet waking, non-REM sleep, transition into REM sleep or quiet REM sleep periods. A percentage (20.4%) of the cells had high discharge associated with phasic REM sleep periods while 8.2% of the cells showed a progressive increase in discharge from waking to REM sleep. Subsequent examination of the distribution of choline acetyltransferase immunoreactive cells in the PRF revealed that cells showing high discharge during REM sleep were not localized near presumed cholinergic neurons. Indeed, we did not find any ChAT immunoreactive somata in the medial PRF, an area which has traditionally been implicated in the generation of REM sleep. These results suggest that while increased discharge of PRF cells may be instrumental to REM sleep generation, these cells are not cholinergic.