In an attempt to understand the dynamic change of the gonadotropin-releasing hormone-pituitary axis during the transitional stage from prepuberty to puberty, we investigated gonadotropin secretory patterns using a highly sensitive assay system and frequent blood sampling technique in children with Turner syndrome aged 5–17 y. Blood samples were collected every 20 min for 24 h in 16 cases, or every 30 min for 9 h (daytime 5 h, nighttime 4 h) in nine cases. Serum LH and FSH were measured by time-resolved fluoroimmunoassay. A 24-h profile of LH and FSH was analyzed by a computerized pulse detection program (PC-PULSAR). As early as 5 to 6 y of age, mean daytime LH concentration was significantly higher than nighttime concentration and pulsatile LH secretion existed throughout the day and night. At about 9 to 11 y of age, corresponding to the early stage of puberty, a dramatic increase in LH concentration and amplitude was observed, and both concentration and pulse amplitude were much higher during the night than during the day. However, these day-night differences became less clear at ages corresponding to late pubertal stages. Pulse frequency of LH secretion remained almost constant throughout the day and night at all ages investigated. As for FSH concentration, a trend similar to that of LH was observed, although day-night differences and age-related changes were less remarkable. Furthermore, pulsatile FSH secretion was detected in only a small number of the cases. These findings suggest that in Turner syndrome the hypothalamic gonadotropin-releasing hormone oscillator is functioning actively with constant frequency before the onset of puberty. Although we know that the day-night rhythm of gonadotropin secretion exists long before the onset of puberty, what controls the day-night rhythm of gonadotropin in the younger girls and the factors responsible for the dramatic increase of gonadotropin around the age of early puberty remain to be determined.