OBJECTIVE: To investigate the effect of Niupo Zhibao Pellet (NPZBP) on the expression of neuronal nitric oxide synthase (nNOS) in the brain of endotoxin-induced shock rats. METHODS: SD rats were randomly divided into normal control group, endotoxin-induced shock model group and NPZBP-treated group. Lipopolysaccharide (LPS) (1.5 mg/kg i.v.) and tsD-galactosamine (D-GalN) (100 mg/kg i.p.) were administered to the rats in endotoxin-induced shock model group, as well as to the rats in NPZBP-treated group after seven-day treatment, to induce the shock. The expression of nNOS in the brain of the rats in each of the 3 groups was measured by immunohistochemical methods. RESULTS: In the 3 groups, nNOS immuno-positive cells distributed widely in layer II, III, IV of the cerebral cortex, the molecular layer of hippocampus, the polymorphic layer of the dentate gyrus, the reticular formation of brain stem, and the molecular, granular and Purkinje cell layer of the cerebellar cortex. The number of immuno-positive cells in the NPZBP-treated group was slightly higher than that of the normal control group, and significantly lower than that of the model group (P<0.05) in many regions of the brain, including cerebral cortex, hippocampus, brain stem and cerebellar cortex. CONCLUSION: NPZBP can inhibit the over-expression of nNOS in wide area of the brain in endotoxin-induced shock rats.