NADPH diaphorase histochemical staining was investigated in rat brain and choroid plexuses. All epithelial cells of the latter as well as some sparse neurons in striatum and cerebral cortex showed strong NADPH reaction product. While staining was homogeneous in neuronal cytoplasm, it was particulate in epithelial cells. Preincubation with EDTA (0.1 mM, 2h) prevented appearance of NADPH diaphorase reaction in neurons but not in choroid plexuses. These data show that in rat brain two forms of NADPH diaphorase are present; they are specifically localized in neurons and choroidal cells, respectively.