The ultrastructural localization of NADPH-diaphorase was studied in the goldfish brain by means of the tetrazolium salt technique. The reaction product was found to be bound to the endoplasmic membranes of neurons in different brain areas. In the synaptic structures both pre and post-synaptic profiles showed the reaction product. Furthermore non-neuronal structures were intensely labeled. Endothelial cells revealed the membranous localization of NADPH-diaphorase and the glial cells of the hypothalamic nuclei and of the paraventricular organ were labeled. In some cases the reaction product was seen also in oligodendrocytes and in microglial cells.