The term serotonergic cell groups refers to collections of neurons in the central nervous system that have been demonstrated by histochemical fluorescence to contain the neurotransmitter serotonin (5-hydroxytryptamine) ( Fuxe-1970 ). The serotonergic cell groups are named B1 cell group, B2 cell group, B3 cell group, B4 cell group, B5 cell group, B6 cell group, B7 cell group, B8 cell group, and B9 cell group ( Felten-1983 ). Since they are for the most part localized to classical brainstem nuclei, particularly the raphe nuclei (histological), they are more often referred to by the names of those nuclei than by the B1-9 nomenclature.