Nitric oxide(NO) is an inorganic, gaseous free radical that carries a variety of messeges between cells including vasorelaxation, neurotransmission and cytotoxicity. NO is produced by a nitric oxide synthase(NOS) which is an enzyme to oxidize the amino acid L-arginine to form L-citrulline and NO. Recently NO production has been described in the respiratory epithelium of the trachea and upper airways. In vitro studies have demonstrated NO production in respiratory epithelial cells induced by exposure to cytokines and an immunohistochemical study using human bronchial biopsies showed NOS to be present in the airway epithelia in normal conditions and increased in asthmatics. The purpose of this study was to determine the presence and distribution of NOS which is an enzyme for NO synthesis in the rat larynx. Immunohistochemical staining using polyclonal anti-endothelial nitric oxide synthase(eNOS) antibody and polyclonal anti-brain nitric oxide synthase(bNOS) antibody was performed to detect NOS in the rat larynx. eNOS immunohistoreactive staining was observed in vascular endothelial cells and bNOS immunohistoreactive staining was observed around acini of submucosal glands. These findings suggest that NOS is present in the laryngeal tissues and NO produced by specific types of NOS may function as a regulator for vasorelaxation and neurotransmission of excitatory amino acid in the larynx. Further study will examine the changing pattern of NOS expressions in specific irratative conditions which can alter the laryngeal reflex mechanism.