With the exception of a report by R.B. Veazey, D.G. Amaral, and W.M. Cowan (1982, J. Comp. Neurol. 207:135-156) that examined the projections of the posterior hypothalamic area in the monkey by using the autoradiographic technique, the ascending projections of the posterior nucleus (PH) of the hypothalamus have not been systematically examined in any species. The present report describes the ascending projections of PH in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L). The major ascending route for PH fibers is the medial forebrain bundle. PH fibers project densely to several subcortical and cortical sites. The subcortical sites are the subthalamus/hypothalamus (zona incerta, the supramammillary nucleus, lateral, perifornical, dorsal, and anterior nuclei/areas), the thalamus (lateroposterior, laterodorsal, parafascicular, reuniens, paraventricular, central medial, paracentral, central lateral and intermediodorsal nuclei), the amygdala (central, lateral, and medial nuclei), the septal area (bed nucleus of stria terminalis, medial and lateral septum), and the basal forebrain (horizontal/vertical limbs of diagonal band nuclei and lateral preoptic area). The cortical sites are the perirhinal, insular, frontal (lateral agranular), prelimbic, and infralimbic cortices. The diversity of PH projections to subcortical and cortical "limbic-related" sites and to several structures with direct input to the hippocampus (supramammillary nucleus, reuniens, paraventricular and laterodorsal nuclei of the thalamus, medial and lateral septum, and perirhinal cortex) suggest that the PH may serve a critical role in various components of emotional behavior, including mnemonic processes associated with significant emotional events.