Decerebrate cats were injected with carbachol into the locus coeruleus (LC) or with carbachol or bethanechol into the dorsal pontine reticular formation (pRF) of one side; recordings were made of the tonic contraction of forelimb extensor muscles of both sides and of their responses to sinusoidal roll tilt of the animal. Both drugs had similar effects when injected into the pRF: a decrease in the tonic contraction of limb extensors and a greatly enhanced amplitude and gain with slightly decreased phase lead in the responses to animal tilt of the forelimb extensor, triceps brachii, ipsilateral to the side of injection. Injected into the LC, carbachol produced a response opposite to the above: it increased the tonic contraction of limb extensors ipsilateral to the side of injection, but decreased the amplitude and gain of the EMG responses of limb extensor muscles to labyrinth stimulation induced by sinusoidal tilt. These findings did not depend on changes in posture since they were still observed when postural EMG activity was maintained constant by appropriate changes in static stretch of the muscle. Moreover, the magnitude of the effects increased in a dose-dependent manner. Results suggest that cholinergic activation of dorsal pRF neurons through muscarinic receptors increases the background discharge of medullary inhibitory reticulospinal (RS) system neurons, thus increasing their modulatory influence. Further, it is postulated that cholinergic activation of LC neurons would cause them to inhibit this tonic facilitatory drive by the pRF. Common to both sites of carbachol injection is the increase in phase lag of the EMG response of limb extensors to animal tilt.(ABSTRACT TRUNCATED AT 250 WORDS)