Pineal secretion of melatonin is hypothesized to suppress gonadal development in both experimental animals and man. A recent report by Silman et al demonstrated a marked decrease in blood melatonin in males but not in females at the time of the onset of puberty.We have developed a gas chromatographic-mass spectrometric method for urinary 6-hydroxymelatonin, which is the major metabolite of melatonin excreted by man. We have previously shown that normal adults exhibit a circadian pattern for urinary excretion of 6-hydroxymelatonin with a total excretion ranging to 6-18 μg/24 hrs and with greater than 80% of the total excretion being from 12 midnight to noon. The urinary excretion of 6-hydroxymelatonin appears to reflect the pineal secretion of melatonin.We measured the circadian excretion of 6-hydroxymelatonin in 24 normal children ages 3-15 years to determine whether the pattern or amount of 6-hydroxymelatonin excreted varies with age and pubertal development and specifically whether a decrease in excretion might accompany the onset of puberty. The normal children showed the same circadian rhythm as adults with the range of total secretion of 8 to 20 μg/24 hours.Our data suggests that the pineal gland in children secretes adult levels of melatonin beginning at the earliest ages studied.