The loci coeruleus of freely moving cats were perfused bilaterally with acetylcholine at a dose of 0.001 microgram/microliter per min, while the animals were recorded polygraphically. The controls consisted of experiments in which no perfusions were done, and experiments in which the loci coeruleus were perfused bilaterally with saline, 1 microliter/min. The acetylcholine produced a sharp inhibition of rapid eye movement (REM) sleep, prolonged the sleep cycle and the REM interval and decreased the number of REM periods and the mean duration of the REM episodes. Total sleep time was increased. This was entirely due to a specific elevation by acetylcholine of deep slow-wave sleep; light slow-wave sleep was not affected. The data support the Hobson-McCarley hypothesis of how the brain controls the REM state, and also suggest that cholinergic stimulation of the locus coeruleus may be important for the shift from REM sleep into slow-wave sleep.