Neuropeptide W (NPW) is produced in neurons located in hypothalamus and brain stem and its receptors are present in the hypothalamus, in particular in the paraventricular nucleus (PVN). Intracerebroventricular (i.c.v.) administration of NPW activated, in a dose related fashion, the hypothalamo-pituitary-adrenal (HPA) axis as determined by plasma corticosterone levels in conscious rats but, at those same doses, did not stimulate the release of oxytocin or vasopressin into the peripheral circulation or alter blood pressure or heart rate. The ability of i.c.v. administered NPW to stimulate the HPA axis in conscious male rats was blocked by intravenous pretreatment with a corticotropin releasing hormone (CRH) antagonist. This suggested an action of NPW in the parvocellular division of the PVN. Indeed, in hypothalamic slice preparations (whole cell patch recording) bath application of NPW depolarized, and increased the spike frequency of the majority of electrophysiologically identified putative neuroendocrine PVN neurons. Effects on membrane potential were maintained in the presence of tetrodotoxin suggesting them to be direct postsynaptic actions on these neuroendocrine cells. Our data suggest that endogenous NPW, produced in brain, may play a physiologically relevant role in the neuroendocrine response to stress.