The concentrations of noradrenaline in individual hypothalamic nuclei and in the median eminence were measured 7-10 days following surgical transections of the lower brain stem or electrolytic lesions of the medullary noradrenaline-containing cell groups. Terminal degeneration in the hypothalamus was studied after the same surgical procedures. Direct, monosynaptic connections between the dorsomedial hypothalamic nucleus and all the noradrenaline-containing cell groups investigated were found. Degenerated synaptic boutons were demonstrated in the median eminence, arcuate, dorsomedial, ventromedial, periventricular and paraventricular nuclei following lesions of the solitary tract and of the lateral reticular nucleus. Biochemical measurements indicate that the pontine-medullary noradrenergic cell groups are the source of hypothalamic norepinephrine. Ascending noradrenergic fibers destined to terminate in the hypothalamus are provided by several cell groups, though the bulk of the NA-fibers seem to originate in the A1-group, in the ventrolateral part of the medullary reticular formation. Most of these fibers join the ventral NA bundle, fewer join the dorsal periventricular tract and several probably also join the dorsal NA bundle. A significant overlap was found in the hypothalamic arborization of the noradrenergic fibers, so that no strict topographical organization seems to be present either in their origin or their termination.