Hypothalamic hypophysiotropic or 'tuberoinfundibular' neurons can be identified by antidromic activation following electrical stimulation on the surface of the median eminence. These neurons appear to be localized within the medial hypothalamic and preoptic areas. Both the source and the nature (i.e. excitatory or inhibitory) of connections of these identified neurons can be examined with electrophysiological methods. Afferent pathways appear to originate in the medial preoptic area, amygdala, hippocampus, lateral septum and retina. Axon collaterals of some tuberoinfundibular neurons engage recurrent excitatory and inhibitory pathways and/or project to the anterior hypothalamic area, medial preoptic area, paraventricular nucleus, amygdala, lateral septum and midline thalamus. Neuropharmacological studies indicate that gamma-aminobutyric acid, catecholamines and possibly some hypothalamic peptides may participate in synaptic transmission in afferent or efferent connections in the tuberoinfundibular system.