The function of ovarian grafts in male rats, castrated within 24 hr after birth (NCmales) was studied by daily examination of smears taken from vaginal grafts. The smear cytology obtained from vaginal grafts in female rats could be correlated exactly with the stage of the ovarian cycle. NC—males of the (R X IDF, hybrid combination appeared to have 5–day cycles only, whereas animals of the R—strain showed besides S—day cycles also cycles with a luteal phase lasting 23–32 days. Injection of Nembutal at different times on the day of proestrus showed that in NC—males a critical period for the ovulation—inducing discharge of gonadotropins existed, similar to that known in females. Daily tests with intact male rats revealed that female—like mating behavior of NC—males could be observed only on the afternoon at proestrus. After a limited number of cycles, ovulations ceased to occur and constant vaginal cornification followed. The grafted condition of the ovary was ruled out as a possible cause of this early anovulatory condition in parallel studies with female rats. NC—males with constant vaginal cornification showed several aspects of female—like mating behavior (lordosis) when exposed to intact male rats. Following mating, corpora lutea were present in the grafts and a short period of normal cyclicity followed again. Corpus luteum formation followed by a short new period with ovarian cyclicity could also be induced with a single injection of 2.5 mg progesterone. It is concluded that female characteristics of ovarian function are present in NC—males during a limited period. It is discussed that this limitation may be due to partial masculinization of the brain by prenatal and early postnatal exposure to androgen. (Endocrinology91: 389, 1972