Previous studies have demonstrated that the epithelium of the respiratory portion of rat nasal mucosa is amply supplied by nerve fibers with immunoreactivities for calcitonin gene-related peptide (CGRP) and substance P (SP), these fibers most likely acting as sensory mediators in the mucosa. The present study demonstrates that some intraepithelial fibers contain a VIP-immunoreactivity whose occurrence in these nerves has previously been neglected. The present study further aims to confirm the occurrence of NO-producing intraepithelial nerve fibers in the rat nasal mucosa and to examine its colocalization with CGRP and with VIP. Double staining methods were used to evaluate the colocalization of NADPH-diaphorase. The reactivity for NADPH-diaphorase and that for CGRP coexisted in only a small part, if any, of the nerve fibers distributed at the basal portion of the epithelium. In the perpendicularly and obliquely oriented transepithelial nerve fibers, both reactivities were clearly demonstrated to be separated in different fibers. VIP immunoreactivity was also present in a part of the intraepithelial nerve fibers of the nasal mucosa, and their entire population was shown to be positive for NADPH-diaphorase. The NADPH-diaphorase-positive reaction was displayed in only a small population of neurons in the trigeminal ganglion, whereas it was seen in numerous neurons in sphenopalatine ganglion, being colocalized with VIP.