This paper summarizes the recent knowledge on the role of melatonin in humans. The first part of the present review considers the following aspects: circadian rhythm of melatonin excretion, melatonin in body fluids, melatonin influence on sexual maturation, changes of melatonin blood concentration during the menstrual cycle and pregnancy, and the influence of melatonin administration on prolactin concentration. Current views on the relationship between melatonin and the hypothalamo-pituitary-adrenal cortex axis, as well as on changes of melatonin concentration during hypothyreosis, hyperthyreosis, and hyperparathyroidism are also discussed.