The distribution of the well-labeled nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) Type I neurons was evaluated in the isocortex of four mammalian species: the Didelphis opossum, the Monodelphis opossum, the rat and the marmoset. In Didelphis opossum, laminar distribution was examined in tangential and non-tangential sections. The density increases from superficial to deep layers of the gray matter. In rats' tangential sections, infragranular and supragranular layers have higher density than layer IV. Cell density measurements in the visual and the somatosensory cortices were compared in tangential sections from flattened hemispheres of the four species. Somatosensory areas were identified histochemically in rat (barrel fields) and marmoset (S1 and S2/PV). In the opossums, areas S1 and S2/PV were identified by multiunit recording. Except in the rat, primary visual cortex (V1) was labeled histochemically by NADPHd and/or cytochrome oxidase. In the four species, cell density in somatosensory cortex was significantly higher than in visual cortex. Taken together these results demonstrate that NADPHd Type I neurons are not homogeneously distributed in the isocortex of these mammals. In conclusion, the tangential distribution of Type I neurons in the sensory areas examined, but not its laminar distribution, was similar in the four species. Given that rats, marmosets and opossums are distantly related species, and that the latter are considered to have more 'generalized' brains, it is conceivable that this pattern of tangential distribution of Type I neurons is a general feature of mammalian isocortex.