To study mechanisms underlying the postural muscle atonia and respiratory depression associated with rapid eye movement (REM) sleep, the cholinergic agonist, carbachol, was microinjected into the pontine reticular formation in decerebrate, spontaneously breathing rats. Carbachol injection led to a simultaneous depression of neck and tonic intercostal EMG activity (lasting 14.5 min +/- 7.6 (S.D.)) and a reduction of the respiratory rate. Phasic inspiratory intercostal activity was not consistently depressed. After a spontaneous recovery from the atonia and respiratory depression, subsequent carbachol injections at the same site produced similar responses. Thus, the decerebrate rat may provide a useful model for studies of the inhibitory neural mechanisms activated by the increased acetylcholine levels in the pons that occur in REM sleep. Pontine carbachol effects in rats differ from those described in cats in a manner analogous to differences in the patterns of natural REM sleep in these two species.