Although there is much evidence that single microinjections of cholinomimetics into the pontine reticular formation (PRF) evokes rapid eye movement sleep (REMS), no study has yet demonstrated whether protracted manipulations of PRF cholinergic levels can produce sustained alteration of this sleep state. In this study, in rats, an indwelling, chronically implanted osmotic mini-pump was used to infuse carbachol, scopolamine, or saline solutions into various brainstem regions or the fourth ventricle for a period of five consecutive days. Throughout the period of pump operation, carbachol infusions chiefly in the PRF produced sustained REMS augmentation primarily during the night cycle, whereas scopolamine produced a sustained decrease in REMS primarily during the day cycle. The findings provide considerable support for a PRF cholinergic hypothesis of REMS generation and regulation and suggest that the alterations in REMS result from a muscarinic receptor mediated change in PRF neuronal activity.