The present study examined the regional localization of corticotropin-releasing factor (CRF)- and 5-hydroxytryptamine (5-HT)-immunoreactive (IR) fibers within the rat dorsal raphe nucleus (DRN) using immunohistochemistry. Additionally, the effects of CRF, administered intracerebroventricularly (0.1-3.0 micrograms) or intraraphe (0.3-30 ng), on discharge rates of putative 5-HT DRN neurons were quantified using in vivo single unit recording in halothane-anesthetized rats. CRF-IR fibers were present at all rostrocaudal levels of the DRN and exhibited a topographical distribution. CRF produced predominantly inhibitory effects on DRN discharge at lower doses and these effects diminished or became excitatory at higher doses. Inhibition of DRN discharge by CRF was attenuated by the nonselective CRF antagonist, DPheCRF12-41 and the CRF-R1-selective antagonist, antalarmin, implicating the CRF-R1 receptor subtype in these electrophysiological effects. The present findings provide anatomical and physiological evidence for an impact of CRF on the DRN-5HT system.