Cultured neurohypophysial cells from human fetuses aged 130–155 days release into the culture medium an active principle which has antidiuretic, hydroosmotic and rat uterine activities. The chromatographic mobility of the active principle, as well as the susceptibility of all of the above activities to tryptic digestion, indicates the presence of a basic peptide identical to arginine vasotocin (AVT). The ability of these cultured cells, which are probably ependymal cells, to release the above activities during 43 days of incubation, suggests that AVT is synthesized and secreted by specialized ependymal cells in the developing human neurohypophysis. Neither in the culture media from anterior pituitaries of the same fetuses nor in control culture media could antidiuretic, rat uterine or hydroosmotic activities be detected. Although the nonincubated neurohypophyses of the same age contain pharmacological activities suggesting the presence of vasopressin, oxytocin and AVT, the neurohypophysial cells cultured in vitro apparently release only AVT into the medium. This supports the suggestion that whereas vasopressin and oxytocin are synthesized in the hypothalamic neurosecretory cells, AVT, on the contrary, appears to be synthesized by ependymal neurohypophysial cells in the developing human neurohypophysis. Consequently, the fetal human neurohypophysis, and presumably the fetal mammalian neurohypophysis, appears not only as a storage site for neurohypophysial hormones but also as an endocrine structure which synthesizes and secretes AVT by ependymosecretio