Histamine-immunoreactive cell bodies are strictly confined to the ventral part of the posterior hypothalamus and the region of the mamillary nuclei. They can be divided into two main cell groups with two major ascending projections and one minor descending projection. The ascending histaminergic fiber tracts innervate almost all regions of the di- and telencephalon with high numbers in for instance the median eminence, the nucleus of the diagonal tract of Broca, the caudate-putamen complex and cortical structures. The arrangement of the histaminergic neuronal system, i.e. a compact cell group with a widespread distribution of fibers, resembles that of other monoaminergic (i.e. serotonergic and catecholaminergic) systems. The vestibular system receives a high innervation of serotonergic fibers, a moderate density of noradrenergic fibers and only scattered histaminergic fibers.