The role of neuropeptide Y (NPY) in the mediation of orexin-induced hypothalamic-pituitary-adrenal (HPA) activation was investigated in the rat. The HPA system was stimulated by intracerebroventricular (i.c.v.) administration of orexin-A or orexin-B (140 or 280 pmol, respectively) and the plasma concentration of corticosterone was used as an index of the degree of activation. i.c.v. pretreatment with NPY antagonist or NPY antiserum (30 min or 24 h before orexin administration, respectively) inhibited the orexin-induced corticosterone release. The inhibitory actions of the antagonist and the antiserum were revealed by the dose-response curve; the highest concentrations practically abolished the HPA activation evoked by the orexins. These data suggest that the HPA system-stimulating effect of the orexins may be mediated by NPY.