The influence of a single intramuscular injection of estradiol benzoate (E2B), 2.5-15 micrograms/kg, on the basal plasma estradiol, LH and FSH and their response to LRH stimulation (50 micrograms/m2, i.v.) performed 20 h after E2B injection, was studied in 50 normal girls aged 6-17 years. It was found that the minimal dose of E2B which caused a significant change in the basal and/or LRH stimulated LH and FSH levels was 10 micrograms/kg. This dose increased plasma estradiol level by 10-35 pg/ml, i.e. still within the normal range for the appropriate pubertal stage of the subjects. The same dose of E2B significantly suppressed the basal FSH level in most of the prepubertal (n = 8), early-pubertal (n = 9) and late-pubertal (n = 14) girls and the FSH response to LRH in most prepubertal girls. The basal LH was suppressed in most pre- and early-pubertal girls, but increased significantly in 50% of the late-pubertal girls. The LH response to LRH was significantly increased in most of the girls at all pubertal stages. It is suggested that endogenous estradiol is an important mediator in the maturation of the hypothalamo-pituitary axis in girls from mid-puberty onwards, and that the low physiological level of estradiol present in adolescence is sufficient to induce a gradual increase in pituitary sensitivity to LRH and a positive hypothalamo-pituitary feedback.