The main goal of the present work was to investigate the pattern of NADPH-diaphorase activity in the somatosensory cortex of the adult mouse. Our results show that this enzyme, which is responsible for the production of the neuronal messenger nitric oxide, is abundant within the neuropil of SmI cortex, revealing the complete pattern of barrel fields. A previous study, however, had reported that NADPH-diaphorase reactivity within the barrels was transient, disappearing after the second postnatal week. We hypothesize that the massive occurrence of NADPH-diaphorase in the barrel fields of the adult mouse brain is related to the potential for plastic changes in the somatosensory cortex that is maintained throughout maturity.