The developmental importance of estrogen and androgen in various cellular aspects of sexual differentiation of the brain will be further defined in organotypic cultures of the newborn mouse hypothalamus/preoptic area and other steroid receptor-containing regions of the central nervous system. The morphological parameters of neuronal differentiation and development will be studied in living and stained preparations at both the light and electron microscopic levels and correlated with metabolic studies of the steroidal effects. Information will also be obtained by immunofluorescence cytochemistry and by 3H-steroid, 3H-thymidine and 125I-autoradiography. These studies are designed to further define the morphological nature of the steroidal effects and to characterize the role of the gonadal steroids in the development of brain regions containing high affinity steroid receptors.