Serum prolactin, follicle-stimulating hormone and luteinizing hormone were determined in 200 girls and 80 boys. The boys have been examined on three occasions at one-year intervals and the girls twice at 1.5-year intervals. In girls, serum FSH rapidly increased in the youngest age groups (7.5-11.5 years), whereas in boys, the increase took place later and the first significant increase was seen between age groups 9.5 and 12.5 years. In girls, a rise in serum LH took place later than that of FSH (between 10.5 and 11.5 years), and LH peaked at 13.0-13.5 years. In boys, the timing in the changes of serum LH closely resembled that of FSH. The girls displayed a significant increase in serum prolactin between 7.5 and 8.5 years, and this was followed by a slow progressive increase. In the group of boys, serum prolactin did not show any significant changes. In girls, there was a correlation between serum LH and body weight, as well as calculated fat amount and body fat percentage early in puberty. There was no correlation between serum LH and chronological or bone age in this age group, which suggests that the correlation found is not due to age-related parallel phenomena.