Abstract: Serotonergic systems play an important and generalized role in regulation of sleep-wake states and behavioral arousal. Recent in vivo electrophysiologic recording studies in animals suggest that several different subtypes of serotonergic neurons with unique behavioral correlates exist within the brainstem raphe nuclei, raising the possibility that topographically organized subpopulations of serotonergic neurons may have unique behavioral or physiologic correlates and unique functional properties. We have shown that the stress-related and anxiogenic neuropeptide corticotropin-releasing factor can stimulate the in vitro neuronal firing rates of topographically organized subpopulations of serotonergic neurons within the dorsal raphe nucleus (DR). These findings are consistent with a wealth of behavioral studies suggesting that serotonergic systems within the DR are involved in the modulation of ongoing anxiety-related behavior and in behavioral sensitization, a process whereby anxiety- and fear-related behavioral responses are sensitized for a period of up to 24 to 48 h. The dorsomedial subdivision of the DR, particularly its middle and caudal aspects, has attracted considerable attention as a region that may play a critical role in the regulation of acute and chronic anxiety states. Future studies aimed at characterization of the molecular and cellular properties of topographically organized subpopulations of serotonergic neurons are likely to lead to major advances in our understanding of the role of serotonergic systems in stress-related physiology and behavior.