Rat/human-corticotropin-releasing hormone (CRH) microinfused unilaterally into the locus coeruleus (LC) of awake, chronically cannulated rats produced intense behavioral stimulation accompanied by a marked decrease of the proliferative response of splenocytes to Con A and LPS and natural killer activity. These effects were specifically prevented by prior administration into the same site of the CRH antagonist (alpha-helical CRH [9-41]). The present results confirm that a strict relationship exists between the CNS and cell-mediated immunity; in addition, they also indicate that CRH produces its behavioral and immune changes by an interaction with specific receptors and that one of the main sites through which CRH exerts these effects is represented by the LC.