Using the fluorescence histochemical technique of Falck and Hillarp, a quantitative microfluorimetric study of the catecholamine fluorescence in the median eminence has been performed. On the basis of morphologic criteria, the median eminence was subdivided into various areas from which the microfluorimetric measurements were made; the subependymal layer, the medial and lateral palisade zone of the rostral and the central and caudal region of the median eminence (for definitions of the various areas and regions, see Anatomical Subdivision). The highest fluorescence intensities were recorded from the lateral palisade zone, indicating that this area has the most dense catecholamine innervation, whereas the lowest fluorescence intensities were recorded from the subependymal layer. Dopamine-beta-hydroxylase inhibition produced by FLA-63, fusaric acid or diethyldithiocarbamate resulted in all cases in a 50-70% reduction of the catecholamine fluorescence in the subependymal layer, whereas only minute effects were observed in the lateral palisade zone. In the medial palisade zone, these treatments generally led to a substantial reduction (30-50%) of the catecholamine fluorescence. Basal hypothalamic deafferentation according to Halasz, or lesioning of the ventral catecholamine bundle, produced an almost complete disappearance of the fluorescence in the subependymal layer, while both procedures were largely ineffective in affecting the catecholamine fluorescence in the lateral palisade zone. On basal hypothalamic deafferentation the catecholamine fluorescence in the medial palisade zone was markedly reduced (40-60%), while the ventral bundle lesions were less efficient in this respect. From the present results it is suggested that the subependymal layer is mainly innervated by noradrenaline nerve terminals and the lateral palisade zone is mainly innervated by dopamine nerve terminals, whereas the medial palisade zone receives a mixed innervation of dopamine and noradrenaline terminals, the dopamine proportion being in the order of 50-75% of total catecholamine content.