Auditory stimulation during REM sleep increases REM sleep time. The purpose of the present work was to determine if the selective muscarinic M-1 antagonist biperiden could modify the effect of the auditory stimulation on REM sleep. Twelve healthy volunteers were divided into placebo and biperiden groups. All the volunteers were studied under sleep laboratory conditions as follows: one acclimatization night, one baseline night, four nights with auditory stimulation either with placebo or biperiden, and two follow-up nights. Biperiden (4 mg) or placebo in identical capsules was administered 1 hour before beginning the sleep recordings. REM sleep time and REM density in the placebo group were increased relative to baseline by auditory stimulation. Biperiden blocked the REM time increase over the three treatment nights and suppressed the REM density increase over all four treatment nights. Biperiden also increased the latency compared to the placebo group. The present findings suggest that M-1 mechanisms are related to REM sleep regulation.