Anatomical aspects of aminergic neurons contained within or projecting to the hypothalamus have been studied with the Falck-Hillarp fluorescence method, the glycoxylic acid technique, Palkovits' "punch-out" system with radioenzymatic assays, electron microscopy (EM), and immunocytochemical localization of the synthesizing enzymes with the light microscope and EM. Dopaminergic (DA) neurons include the tuberoinfundibular and incertohypothalamic groups. Serotonergic (5-HT) neurons project largely from the raphe nuclei, but there are indications of some intrinsic 5-HT cells. Noradrenergic input to the hypothalamus comes almost completely from lateral tegmental brain stem neurons via the ventral adrenergic bundle, but the paraventricular, dorsomedial, supraoptic and periventricular nuclei receive some fibers directly from the locus coeruleus through the dorsal bundle. EM studies reveal aminergic boutons wiwh dense cored vesicles, but specific synaptic profiles with thickened postsynaptic membranes appear to be limited in number, and it has been proposed that the amines may commonly be released from boutons without true synaptic endings. Electrophysiological studies related to neuroendocrine phenomena reveal stimulatory and inhibitory responses to amines and to activated aminergic neurons. These effects may be transmitted via inhibitory interneurons. Functional recovery from the chronic loss of 85% of hypothalamic norepinephrine suggests that the catecholamine may serve as a modulator rather than as an essential link in the chain of neuroendocrine cause-effect relationships.