The effects of microinjections of a cholinergic agonist, carbachol (0.2 micrograms/0.2 microliters), were examined on a population of presumptive cholinergic mesopontine PGO-on neurons that presents a tonic pattern of discharge during waking and exhibits short spike bursts preceding the onset of dorsal lateral geniculate PGO waves during paradoxical sleep and slow wave sleep just prior to it. PGO-on neurons were activated antidromically by the stimulation of the dorsal lateral geniculate, pulvinar and/or medial and intralaminar thalamic nuclei. They were all characterized by a long spike duration and a slow conduction velocity. Microinjections of carbachol near unit recording sites in freely moving cats induced a complete suppression of the spontaneous tonic activity during waking, but did not suppress the spontaneous phasic burst activity during sleep. Carbachol microinjections also resulted in a marked reduction in responsiveness of PGO-on neurons to orthodromic stimulation. These spike depressant effects lasted for approximately 90-120 min and were reversed completely by a local or systemic administration of atropine sulfate. These findings point to a direct inhibition of central cholinergic PGO-on neurons via a muscarinic autoreceptor and a difference in the mechanisms underlying the generation of tonic and phasic burst activity of PGO-on neurons occurring during waking and sleep.