Using NADPH-diaphorase(NADPH-d)histochemistry combined with immunocytochemistry,we investigated the coexistence of nitric oxide synthase(NOS)with serotonin(5-HT),tyrosine hydroxylase(TH)and acetylcholine(ACh),respectively,in some regions of the rat brain;In the dorsloateral subgroup of the dorsal raphe(DR)nucleus,most of the NOS neurons displayed 5-HT immunoreactive(5-HT-IR),while more than half of the serotoninergic neurons were not NADPH-d positive. A large number of NOS neurons presented in the ventromed...