The fine structure of pituicytes was investigated between 15 days of fetal life and 120 days postnatum. Prior to the penetration of neurosecretory axons into the neural lobe a uniform population of undifferentiated pituicytes is presented. Coincidental with the first appearance of neurosecretory axons in the neural lobe at 16 1/2 days of fetal life is the beginning differentiation of those pituicytes into two varieties, an active and an inactive one. The active variety predominates and has all of the morphologic characteristics of an active secretory cell, especially during axonal growth in the neural lobe; between 16 1/2 days and birth most of the undifferentiated pituicytes have differentiated into active pituicytes. The inactive variety is rarely encountered before birth; the classification as an inactive cell is based on the lack of organelles involved in secretion. After adult conditions are reached between 25 and 30 days postnatum, the active pituicytes continue to prevail in the neural lobe, although they are less active than in the developmental period as judged by morphologic criteria. The inactive pituicytes increase in number with increasing age. In addition to the pituicytes two other cells are described, microglial cells and a cell whose exact nature remains to be determined.