Six cats were chronically implanted with a standard set of sleep-scoring electrodes and bilateral stainless-steel guide tubes for microinjection of drugs in the peribrachial area (PBL). Pretreatment of drug injection sites in the PBL with the M2 antagonist methoctramine blocks both the immediate triggering of ponto-geniculo-occipital (PGO) waves and the later prolonged enhancement of REM sleep that is induced by carbachol. These results support the hypothesis that the carbachol effects are mediated via the M2 muscarinic receptor that is known to be present in the PBL.