With an antibody specific for L-histidine decarboxylase (HD) in combination with immunohistochemical techniques and retrograde fluorescent tracing, the morphology, distribution, and projections of the histaminergic neurons of the posterior hypothalamus were studied in the adult male rat. Magnocellular neurons, situated on both sides of the mammillary recess and close to the ventral surface of the brain rostral and caudal to the mammillary bodies, were found to contain HD-immunoreactivity (HD-i). In addition to these magnocellular neurons, a substantial number of small and medium-sized neurons were immunostained, as were strands of cells of all sizes bridging the HD-i cell groups. A detailed mapping of the HD-i cells in frontal, sagittal, and horizontal sections showed that these neurons make up one continuous cell group, defined as the tuberomammillary nucleus (TM). This nucleus can be divided into several subgroups. Thus, approximately 600 HD-i neurons situated on each side of the mammillary recess compose the medial subgroup of the TM (TMM). The ventral subgroup of the TM (TMV) consists of some 1,500 neurons situated at the ventral surface of the brain, rostral (TMVr) and caudal (TMVc) to the mammillary bodies. The TMM contains a greater proportion of parvicellular neurons compared to the TMV. About 100 HD-i cells are scattered within the lateral hypothalamic area, the posterior hypothalamic region, the perifornical area, the supramammillary nucleus, and the dorsomedial hypothalamic nucleus. These cells are collectively named the diffuse part of the TM (TMdiff). The morphological differences between the TMM and the TMV did not signal differences in the efferent connections of these subgroups. Thus, single injections of the fluorescent tracer Fast Blue into different regions of the brain, including the spinal cord, resulted in retrograde labeling of HD-i neurons, which were distributed throughout the TM with no discernible topographic pattern. More specifically, each subgroup of the TM contributed projections innervating or passing through a large number of brain regions, including the olfactory bulb, hippocampus, caudate nucleus, paraventricular and supraoptic nuclei of the hypothalamus, cerebellum, tectum, medulla, and spinal cord. The widespread projections of the HD-i neurons contrasted to the more specific projections observed from non HD-i neurons in cell groups situated around the TM. Taken together, these findings suggest that the HD-i cells of the posterior hypothalamus constitute one major nucleus, the TM, and that this nucleus may be subdivided into three components, each of which has diffuse projections throughout the neuraxis.