A six day long increase in rapid eye movement (REM) sleep followed the unilateral microinjection of a single dose of the cholinergic agonist drug carbachol into the brain stem of cats. Effective drug injection sites were localized to the pontine peribrachial region containing cholinergic choline acetyltransferase (ChAT) labeled neurons. At the peak of the effect, which occurred 24-28 h post-injection, the relative amount of time devoted to REM sleep tripled, resulting in an absolute time increase from 3.12 to 11.28 h REM sleep per day. This pronounced and prolonged REM sleep increase was associated with marked enhancement of ponto-geniculo-occipital (PGO) waves and with PGO burst cell activity unilateral to the site of injection.