Electrolytic lesions were placed in the hypothalamus of two-day-old female rats. Destruction of the mediobasal part of the preoptic area resulted in persistent vaginal estrus starting on the day of vaginal opening, while lesions placed laterally in the preoptic-anterior hypothalamic area did not interfere with normal cycles. Therefore, the mediobasal hypothalamus is capable of undergoing maturation without any postnatal influence from at least the mediobasal part of the anterior hypothalamus. Destruction of the anterior wall of the third ventricle also caused persistent or prolonged vaginal estrus preceded by normal cycles. The relationship between the loci of lesions and the occurrence of sexual cyclicity was discussed.