Nitric oxide(NO) is an inorganic, gaseous free radical that has the function of vasorelaxation, and cytotoxicity and it is produced by a nitric oxide synthase(NOS) which is an enzyme to oxide the amino acid L-arginine to citrulline and NO.NO, a simple gas that serves as a neurotrans-mitter and second messenger in the CNS, has been proposed to play a role in olfactory transduction. The purpose of this study is to investigate the localization of NOS in the olfactory system of rats. Male Sprague-Dawley rats were used and NOS was detected by immunohistochemistry using anti-bNOS antibody. bNOS immunopositive cells were highly localized in main and accessory olfactory bulb. However, in vomeronasal organ, bNOS immunopositive cells were localized exclusively in the peripheral region of blood vessel and in olfactory epithelium, NOS immunopositive cells were localized in submucosal olfactory nerve fibers. The characteristic pattern of localization for NOS in the olfactory system suggests that the NO may participate in neuronal processing of olfaction in rat's olfactory system.