The present study investigated if NOS positive neurons localized in regions related to defensive reactions are activated after exposure to an innate fear stimulus (a live cat). Male Wistar rats were exposed to a live or a toy cat for 10 min and 2h later had their brains removed and processed for c-Fos immunohistochemistry (a marker of neuronal functional activation) and NADPH-diaphorase (NADPH-d; used to detect the presence of NOS neurons) histochemistry. Cat exposure induced a small (11%) to moderate (50%) significant increase in the percentage of double-stained cells (c-Fos+NADPH-d positive neurons) in the anteromedial bed nucleus of stria terminalis (BSTMA), medial amygdala (MeA), parvocellular paraventricular (pPVN), lateral (LH) and dorsal premammillary (PMd) hypothalamic nuclei, dorsolateral periaqueductal grey (dlPAG) and dorsal raphe nucleus (DRN). This increase was attenuated in the PMd, DRN and dlPAG by i.c.v. injection of AP7 (5 nmol/2 microl), an NMDA receptor antagonist. The drug increased the percentage of time the rats remained close to the cat in the observation box. The results suggest that exposure to a live predator activates neurons containing NOS in brain areas related to defensive reactions. They also indicate that this effect probably involves activation of NMDA glutamate receptors.