We have studied the karyometric development of the ependymocytes of the median eminence and ependyma adjacent of the arcuate nucleus in a control group of male albino mice from the 5th to the 160th postnatal day, and in other two experimental groups of animals that were castrated at two different days: the first and the 20th day of life. We have found differences in the spontaneous development of both ependymocytes showing the median eminence ependyma a more closely relationship to changes of gonadal hormone levels around puberty. In both zones, the response of the ependymocytes to neonatal castration was clearly more significative than that obtained after prepuberal castration, with lower values in the castrated animals than in the control mice. We suggest that this could be related to nervous and hormonal mechanisms.