Previous studies from our laboratories showed that stimulation of the pontomesencephalic reticular formation resulted in two distinct changes in masseteric reflex excitability which were dependent on the behavioral state of the animal (Chase, M. H., and M. Babb. 1973. Brain Res. 59: 421–426). During wakefulness and quiet sleep, reticular stimulation resulted in an increase in reflex excitability. However, during active sleep, the identical stimulus delivered to the same reticular site led to profound reflex suppression. This phenomenon was termed “response-reversal.” The present study was designed to explore the presence of state-dependent control of motor excitability at rostral mesencephalic and pontine levels of the brain stem in chronic freely moving cats during sleep and wakefulness. Conditioning stimulation of mesodiencephalic sites induced only slight reflex facilitation or was without effect during wakefulness and quiet sleep; however, a dramatic suppression of reflex excitability was evoked with the identical stimulus during active sleep. At the level of the pontomesencephalic junction an effective region for “response-reversal” was found to coincide with the nuclei reticularis mesencephali and pontis oralis. The major effect resulting from stimulation of sites surrounding this region occurred exclusively during active sleep and consisted of reflex suppression. During active sleep, from all sites at both levels of the brain stem, only reflex suppression was obtained in conjunction with conditioning stimulation. These findings are discussed in terms of a model of state-dependent regulation of motor activity which accounts for the emergent capability of widespread regions of the brain stem to suppress reflex excitability solely during active sleep.