Nitric oxide(NO) is a noxious, stable, inorganic free radical gas. Recent work in several disciplines has cinverged to establish NO as a non-adrenergic, non-cholinergic neurotransmitter in the central and peripheral nervous system and a major messenger molecule in immune system and blood vessel dilatation. NO is synthesized by nitric oxide synthase(NOS, NADPH-diaphorase) which I an enzyme to oxidize the amino acid L-arginine to form L-citrulline and NO. In nasal system, NO is thought to act as an endothelium-derived relaxing factor(EDRE) in the nasal mucosa as well as a neurotransmitter in peripheral nerve and ganglion cell of nose. The purpose of this study was to determine the existence and distribution of constitutive NOS(cNOS) in the nasal septal mucosa of the rat by histochemical method and to estimate pixel count of NOS positive nerve fiber by image pro plus program. NOS positive nerve fibers were observed around vascular endothelium, gland, nerve fibers, and ganglions. NOS activity was most high intensively oberved in ganglion cell compared with other site. Average pixel count of NOS positive nerve fibers were 217. These results suggest that NOS is present in nasal tissue and NO may be related to regulation of nasal blood flow, secretion of gland, and neurotransmitter of the nose.