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 and L-arginine to form L-citrulline and NO. In the peripheral auditory system, NO is thought to act as an endothelium-derived relaxing factor in the lateral wall as well as a neuromodulator for L-glutamate, excitatory neurotransmitter, in spiral ganglion cells of cochlea. The purpose of this study was to determine the presence and distribution of NOS which is an enzyme for NO synthesis in the cochlear tissue. Immunohistochemical staining using polyclonal anti-endothelial nitric oxide synthase(e-NOS) antibody and polyclonal anti-brain nitric oxide synthase(b-NOS) antibody was performed to detect NOS in the rat cochlea. NOS immunohistoreactive staining was observed in stria vascularis, inner hair cell and spiral ganglion cell. e-NOS immunoreactivity was more intense in stria vascularis, whereas b-NOS immunoreactivity was the most intense in spiral ganglion cell compared to other site. These results suggest that NOS is present in the cochlear tissues and NO produced by specific types of NOS may function as a regulator for vasorelaxation and neurotransmission of excitatory amino acid in the cochlea. Further study will examine the changing pattern of NOS expressions in specific conditions (noise overstimulation or hypoxia) which can alter the cochlear function.