The distribution of reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-D) was mapped histochemically in the forebrain of Rana pipiens, the leopard frog. Intense staining was observed which was strikingly restricted to certain nuclear groups and fiber tracts. The densest concentrations of NADPH-D stained cell bodies and fibers were observed in the granule layer of the accessory olfactory bulb and in the ventral aspect of the lateral pallium. Intense staining has also been reported in the presumed mammalian homologues of these regions. Less densely packed clusters of intensely stained neurons were found in the striatum, the anterior entopeduncular nucleus, the olfactory tubercle and the pars lateralis of the amygdala, whereas the preoptic region and the medial septum exhibited dense accumulations of lightly stained cells. Several fiber systems or terminal fields could be detected, including a ring of heavy staining which enclosed the striatum and an apparent terminal field in the lateral part of the medial pallium. A prominent compact tract, which may be homologous to a component of the stria terminalis of mammals, could be also followed from the ventral portion of the lateral pallium to the infundibular hypothalamus.