Intact females without neonatal hormone treatment served as control animals. A second group of female rats received 1.25 mg testosterone propionate subcutaneously on the first day of life causing in these genetic females a male-type brain differentiation. Male rats orchidectomized on the 75th day of life served as male "control" animals, since there was no lack of androgen during the neonatal phase of brain organization. A fourth treatment group consisted of males which had been orchidectomized on the first day of life representing males with female-type differentiated brains. The experimental animals belonging to the Sprague-Dawley-derived strain of our laboratory were kept under standard conditions for food, illumination and temperature.