The localization of nitric oxide synthase (NOS) and its cofactor, nicotinamide-adenine dinucleotide hydrogen phosphate (NADPH)-diaphorase, was examined in the nasal mucosa of the rat by immunohistochemical and histochemical methods. In addition to cryostat sections, whole mount preparations were used to examine the distribution of nerves. Both in the nasal mucosa and in associated ganglia, the distribution of NOS-immunoreactive nervous structures essentially corresponded to that of NADPH-diaphorase-positive ones. The NOS-immunopositive nerve fibers in the respiratory area of the nasal mucosa were distributed around blood vessels and in submucosal glands. Part of the respiratory area was supplied with intraepithelial arborizations of the immunopositive fibers. The epithelial cells in the respiratory area were NADPH-diaphorase positive but NOS immunoreactivity negative. In the olfactory area, the NADPH-diaphorase- and NOS-positive nerve fibers were restricted to blood vessels located deep in the submucosa. Throughout the nasal mucosa, arterial endothelium was NADPH-diaphorase positive but NOS immunoreactivity negative. Both NOS immunoreactivity and NADPH-diaphorase activity were found in major populations of neuronal somata in the sphenopalatine ganglion. The present study provides the direct evidence supporting the notion that nitric oxide is richly produced in autonomic nerves of the nasal mucosa derived from the sphenopalatine ganglion.