In female rats treated with 100 µg estradiol benzoate on the first day of life, the following data were obtained in adult or juvenile life: (1) predominance of vaginal estrus; (2) significantly enhanced male and completely suppressed female sexual behavior associated with an increased ‘attractiveness’ to the male test partners; (3) hypogonadotrophic hypogonadism; (4) sterility; (5) a significant increase of postpuberal body growth; and (6) non-evocability of the ‘positive estrogen feedback effect’. These findings are explained by a paradoxical influence of estrogen on the organization of the hypothalamus.