Pubertal development of 200 normal girls, 7--17 years of age, was investigated in a partly longitudinal manner with two examinations 1.5 years apart. Samples from postmenarchal girls were taken on days 6--9 and 20--23 of the menstrual cycle. Serum pregnenolone, progesterone, 17-hydroxyprogesterone, dehydroepiandrosterone, androstenedione, testosterone, 5 alpha-dihydrotestosterone, androsterone, oestradiol and cortisol as well as ACTH, FSH, LH and prolactin were measured radioimmunologically and were related to bone age, breast and pubic hair developmental stages, and gynaecological age. In the samples of premenarchal girls as well as the follicular phase of postmenarchal girls the concentration of all the steroids increased with age. Of all the steroids measured, serum dehydroepiandrosterone and pregnenolone displayed the earlier increase, from the youngest age group of 7.5 years onwards. Serum oestradiol testosterone and androstenedione in creased rapidly from the bone age group of 9.5 years (subjects 9.0--9.9 years of age) onwards, in close association with the appearance of the first physical signs of puberty. A marked increase in these three steroids continued until 13.5 years, the age at which menarche took place. Menarche was followed by a plateau of 1--2 years duration and then a second increase took place up to the two oldest age groups (17.5 and 18.5 years bone age), a trend seen in the follicular phase levels of all the steroids measured. The 5 alpha-dihydrotesterone/testosterone ratio decreased with increasing testosterone concentration. Serum oestradiol, testosterone, androstenedione, dehydroepiandrosterone and FSH showed no overlapping in the 2.5--97.5% range of concentrations and androsterone and LH in the 16--84% range between prepubertal and postmenarchal subjects. Pregnenolone, progesterone, 17-hydroxyprogesterone, 5 alpha-dihydrotesterone, cortisol, ACTH and prolactin overlapped even in the 16--84% range between these two groups of subjects. In postmenarchal girls, about 80% of the cycles were anovulatory in the first year after menarche, 50% in the third and 10% in the sixth year. The background of the majority of the anovulatory cycles seems to be a physiological variant of the pattern seen in the polycystic ovary syndrome: the levels of testosterone, and androstenedione and LH were increased in anovulatory cycles compared to ovulatory ones.