Effects of estradiol on the survival of cultured fetal rat amygdala neurons were estimated to assess a possible organizational action of the sex steroid on the developing amygdala tissue. Dissociated 17-day fetal amygdala cells were cultivated initially in a serum-containing and then in a serum-free defined medium. The survival of the cells in the serum-free medium was highly enhanced when supplemented with estradiol at the concentration of 10 ng/ml. Predominant cell populations of the culture were identified as neuronal cells by the tetanus toxin labeling method. The results support the idea that sex steroids play a role in the brain sexual differentiation by enhancing the neuronal survival in the developing amygdala tissue.