Iodinated melatonin was used to study melatonin sites of action in brain. Iodomelatonin mimicked the effects of melatonin on reproductive development in Djungarian hamster fetuses. 125I-melatonin injected into the dam was recovered from fetal brain. In vitro autoradiographic studies revealed a remarkably discrete distribution of competitive 125I-melatonin-binding sites in the fetal brain, with binding in median eminence/arcuate nucleus area greater than suprachiasmatic nucleus greater than pineal gland much greater than anterior pituitary gland much greater than preoptic area. 125I-melatonin promises to be a useful tool for understanding the sites and mechanism of action of melatonin.