In the female rat, the onset of reproductive capacity is signaled by the first ovulation, an event usually associated with vaginal opening. First ovulation is induced by an abrupt, proestrous-type surge of gonadotropins which in all probability results from the expression of estrogen positive feedback. Central nervous-system-pituitary sensitivity to the feedback develops gradually and quantitatively as the animal matures. By contrast, sensitivity to estrogen negative feedback appears to decrease (abruptly) only after first ovulation has occurred. Instrumental in the process of maturation of estrogen positive feedback are an enhanced capability of the hypothalamus to release LHRH (luteinizing hormone-releasing hormone) and an increase in responsiveness of the ovaries to gonadotropins. While serum gonadotropins seem to change very little during the days antecedent to the first preovulatory surge, the somatomamotrophic hormones prolactin (PRL) and growth hormone (GH) become unambiguously elevated. One of the mechanisms by which PRL (and perhalps GH) appears to participate in the maturational process that lead to puberty is by enhancing ovarian responsiveness to gonadotropins. Prolactin exerts part of this effect by increasing luteinizing hormone (LH) receptor content of granulosa cells. In addition, PRL can also act centrally to advance puberty by a mechanism still poorly understood.