Objective:To investigate whether neuronal nitric oxide synthase(nNOS) neurons in the hypothalamic paraventricular nucleus(PVN) and supraoptic nucleus(SON) were involved the central pathways of the arterial baroreflex in rats. Methods: Immunocytochemical method for Fos combining with nicotinamide adenine dinucleotide phosphate diaphorase(NADPH-d) histochemical technique for nNOS was used to localize the Fos and NADPH expressions in the subdivisions of the PVN and SON following sinoaortic baroreceptor denervation(SAD). Results:It was found that distinct number of Fos positive neurons coexisting with NADPH-d in the parvocellular subdivisions, magnocelluar subdivisions of the PVN and the whole SON. The percent of Fos/NADPH-d double-labeled neurons in the above regions was 6.8%, 72.1% and 47.1% respectively. Only few Fos positive neurons were observed at the above regions in the sham-operated and intact control rats, and no NADPH-d/Fos double-labeled neurons were detected. Conclusion:The nNOS neurons in the PVN and SON are activated by the SAD specifically, which highly indicates they may take part in the central cardiovascular regulation mediated by the arterial baroreflex.