The question of whether maternal melatonin could be transported in milk to suckling rats was investigated because melatonin is probably not produced in these animals during the first 10 days of life. [3H]Melatonin was found to be rapidly transferred from the maternal circulation into lactating mammary tissue, and the stomach of each suckling rat was found to contain [3H]melatonin. To study the fate and tissue distribution of [3H]melatonin originating in the neonatal stomach, suckling rats were given [3H]melatonin by stomach tube; [3H]melatonin was recovered from plasma and seven tissues, including brain, 15 and 60 min later. The general tissue distribution of [3H]melatonin was similar to that found in adult rats. The major [3H]melatonin metabolites in the urine of suckling rats, as the adult rats, were the conjugates of 6-hydroxy-melatonin.