Melatonin from the pineal gland is best known for its regulation of the circadian rhythm. However, melatonin synthesis also occurs in other organs, one of which is the thymus. Melatonin synthesis changes with age. To assess whether the thymic melatoninergic system shows changes related to development, Cruz-Chamorro et al. studied the enzyme expression required for melatonin synthesis as well as the nuclear and membrane melatonin receptors in normal human thymuses from children of different ages. They found that the thymic melatoninergic system is present and especially active from the first days of human life and its expression is inversely correlated with the age of the human thymus.