Puberty is characterized by a progressive maturation of the hypothalamus-pituitary gonadal axis which, in girls results in menarche. The first menstrual cycles are usually irregular and anovulatory, and the subtle positive and negative regulation of sex steroids on the hypothalamus-pituitary axis has probably not reached adult maturity. An investigation has been carried out in 99 normal adolescent girls, divided into 3 groups: 1-2, 3-4 and 5 years after menarche, by measuring basal hormonal values as well as the responses to LHRH and TRH during the follicular and luteal phases. Basal FSH and LH values reached adult levels after the second year of menarche, while FSH and LH responses to 50microgram LHRH showed a regular and progressive increase from 1 to 5 years post-menarche, resulting, in the 5-year group and in spite of the half dose received, in definitely higher FSH and LH responses than those observed in the adult women after 100 microgram LHRH. This enhanced pituitary responsiveness to LHRH is due to still progressively increasing estradiol and progesterone secretions, the latter hormone remaining still lower than in the adults. Basal prolactin levels were significantly higher than those found in adult women with a slightly increased prolactin response to TRH and an exaggerated one of TSH, with normal T3 and T4 levels. These data show that from the onset of menarche to the complex and subtle adult menstrual cycle regulation, there is a continuing maturation of the hypothalamus-pituitary axis of the gonads which lasts approximately 5 years. It is characterized by increasing E2 secretion, low progesterone secretion and slightly increased prolactin levels, with a frequently impaired luteal phase. The enhanced pituitary sensitivity to releasing hormones is due to the positive feedback mechanism of E2 which is not yet associated with adequate progesterone secretion for a negative feedback, as in adult women. Thus, adolescence is still a maturation period, the onset of ovulation being the final step in this development.