The modulation of neuronal activity by the gas nitric oxide is one of the most novel discoveries in neuroscience. In the auditory pathway, the highest expression of nitric oxide synthase is found in the inferior colliculus (IC), an important center for the convergence of parallel ascending pathways traveling in the brainstem, and descending projections from the auditory cortex. Here we use immunocytochemistry with an antibody for neuronal nitric oxide synthase (nNOS), or NOS Type 1, to map the distribution of nNOS expression in the IC of the guinea pig. The results show that nNOS is differentially expressed by both cell bodies and neuropil across its different subdivisions. The highest levels of neuronal staining are seen in the dorsal and lateral cortices, and the commissural nucleus, making them readily distinguishable from the ventro-lateral part of the central nucleus where nNOS expression in neuropil and somata is minimal. Dorso-medially, and caudally, however, the region of nNOS expression extends from the dorsal cortex into the area normally designated as the central nucleus, and nNOS is expressed by neurons characteristic of this subdivision. Our findings support the idea of a gradual transition in cell properties rather than a distinct boundary between the central nucleus and the dorsal cortex. This transition zone may provide a cytoarchitectonic substrate for functional interaction between these two subdivisions.