The effects of maternal melatonin on fetal and neonatal melatonin and dopamine D1 receptor systems in the central nervous system, mainly in the suprachiasmatic nuclei (SCN), were investigated after pinealectomy of rats at day 7 of pregnancy. 125I-labelled iodomelatonin injected intravenously into the pregnant rats (at day 21) was transferred in considerable amount into the fetal circulation. In vitro autoradiography data demonstrated an increase in the melatonin binding activity in the fetal (embryonic day 21) and early postnatal SCN (postnatal day 3) caused by maternal pinealectomy. This upregulation of the melatonin receptor in the SCN was then normalized after the melatonin system of the neonate started to work. The pregnant rats themselves did not show such a change in their melatonin receptors in the SCN following pinealectomy. Dopamine D1 receptor binding was affected by pinealectomy exclusively in the SCN of fetal and neonatal rats as well as in that of mothers. These results clearly indicate that the fetal circadian clock in the SCN is controlled and prepared before birth to some extent by maternal melatonin rhythm.