Neurons in the lateral basal forebrain which discharge selectively during slow-wave sleep have been identified as projecting to the midbrain reticular formation. Such cells have been hypothesized to participate in sleep-related changes in reticular formation excitability. The effects of stimulation at these sites on reticular formation single unit discharge in behaving cats was quantified in the present study. Effects were compared to those evoked from medial basal forebrain sites, including the preoptic area/anterior hypothalamus. Stimulation at all of these basal forebrain sites typically produced biphasic responses, consisting of a brief, short-latency excitation, followed by discharge suppression. Stimulation at lateral sites resulted in significantly longer periods of discharge suppression compared to stimulation at medial sites. These results support the hypothesis that laterally situated basal forebrain neurons participate in the regulation of behavioral state, in part, via descending modulation of midbrain reticular formation arousal mechanisms.