Melatonin, the main hormone secreted by the pineal gland at night, plays a major role in regulating reproductive physiology in seasonal breeders and influences the age of sexual maturation in laboratory rodents. In humans these relationships are less clear. Evidence supporting a melatonin-reproductive hormone relationship relies on findings of abnormal melatonin secretion in disorders of the reproductive system and on pathologies of the pineal gland which are associated with clinical abnormalities of the reproductive hormones. Normal melatonin rhythms are closely related to those of the reproductive hormones during infancy and reciprocally correlated during puberty. The demonstration of melatonin receptors in the brain and in reproductive organs, together with the localization of sex hormone receptors in the pineal gland, further strengthen these relationships. However, it is not yet clear that these correlations are functionally related, as data on the antigonadal effects of exogenous melatonin on the reproductive hormones are not conclusively established.