We investigated the ontogeny of melatonin synthesis during fetal maturation by measuring the melatonin (MLT) and 6-hydroxymelatonin sulfate (MLTS) excretion in the urine of male infants aged 2-7 days and gestational age 26-42 weeks. We found a negative correlation between advancing gestational age and the MLT and MLTS excretion expressed as total 24-h amount, ratio of 24-h amount to creatinine and ratio of 24-h amount to body surface area. The ratio of MLT to MLTS was found to be about ten times higher in the study group than in prepubertal children, which might reflect the immaturity of hepatic sulfation capacities. The total amount of excreted MLT and MLTS was only one-tenth the prepubertal values. No day/night differences in MLT and MLTS excretion could be detected. We conclude that the fetal pineal gland is capable of a limited melatonin synthesis from the 26th week of gestation onwards, with decreasing values reaching its nadir around term. This indicates that the amount of fetal MLT excretion is not determined by synthesizing capacities of the pineal gland but by the development of neural connections to the pineal gland.