This investigation has focused upon a set of neuroanatomical correlates that underscore functional changes in the median eminence of the fetal baboon Papio anubis. Eight fetal primate brains were harvested at mid-gestation (100 days post-coitus) and prepared for routine light and transmission election microscopic examination following ventriculo-cisternal perfusion with high osmolarity aldehyde fixatives. The median eminence and other adjacent circumventricular organs (CVO) were blocked and embedded in epon. Routine transmission electron microscopic examination revealed discreet regional differentiation and zonal maturation of the fetal baboon median eminence at 100 days post-coitus. The ependymal and hypendymal zones were anatomically separate from the underlying fibrous and palisade zones of the median eminence. The dominant cell type of the ependymal and hypendymal zones were tanycytes whose apical surfaces constituted the floor of the third cerebral ventricle. The distal processes of tanycytes terminated upon the abluminal basal lamina of well developed perivascular spaces in the contact zone of the fetal median eminence. Numerous axon profiles that harbored both dense core and microvesicles were also observed to terminate upon the system of perivascular spaces that contained numerous fenestrated capillaries. Sharing this common perivascular system were cells of the pars tuberalis that contained numerous secretory inclusions. These data strongly suggest that by mid-gestation the fetal primate median eminence is highly differentiated and may be capable of a wide range of functional activities in response to changes in the maternal-placental environment. The ultrastructural correlates of an active neuroendocrine axis are apparent at this period of development.