The distribution of reduced nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase activity was histochemically investigated in the Japanese quail brain. This enzyme is now considered responsible for the synthesis of nitric oxide, a novel neural messenger whose distribution has not been described in the avian brain until now. The histochemical technique provides a simple and reliable method for staining selected populations of neurons throughout the avian brain. In the telencephalon several regions showed heavily stained NADPH-diaphorase positive neurons and processes. In particular the paleostriatal-paraolfactory lobe complex showed the greatest presence of both positive cells and processes. Neurons and processes were also observed in several regions of the hyperstriatum as well as in the archistriatal nucleus taeniae. Some regions, such as the ectostriatum and the hippocampus, had no positive elements. In the diencephalon, the magnocellular hypothalamic system, which in mammals shows NADPH-diaphorase activity, did not show any particular accumulation of reaction product. On the contrary, retinorecipient areas, such as the visual suprachiasmatic nucleus and the lateral geniculate complex, displayed a composite structure of both positive neurons and processes. The brainstem revealed a large NADPH-diaphorase positive population extending through the tegmental nuclei to the locus coeruleus and subcoeruleus. A complex organization was also observed in the optic lobe, where fusiform elements were distributed within the stratum griseum and superficialis of the tectum. In the medulla, a dense terminal field was observed at the level of the nucleus of the solitary tract, whereas scattered neurons were located within the reticular nuclei. Although the staining of neurons and tracts was highly selective, the positive cells did not correspond to any single known neurotransmitter, neuropeptide, or neuroactive molecule system. Several sensory pathways were heavily stained for the NADPH-diaphorase, including part of the olfactory, visual, and auditory pathways. The findings of the present study reveal that the NADPH-diaphorase-containing systems in the avian brain are organized according to a pattern comparable, because of its complexity, to that observed in mammals. However, important interspecific differences suggest that this novel neural system might be involved in diverse tasks.