The presence of nitric oxide synthase in spinal cord and dorsal root ganglia was investigated by immunohistochemistry using antibodies against the constitutive neuronal form of nitric oxide synthase (NOS). NOS immunoreactivity was present in both man and rat with similar distribution, being present in primary sensory neurons of dorsal root ganglia and their afferent terminals in dorsal horn of spinal cord. NOS immunoreactive interneurons were found in the superficial layer of the dorsal horn, around the central canal and in the intermediolateral cell column. NOS immunoreactivity was also present in numerous motoneurons in the ventral horn. The widespread distribution of NOS in both sensory and motor nervous system is indicative of the involvement of nitric oxide in different neural functions.