The hormone melatonin was first identified about 30 years ago as a secretory product of the pineal gland. In mammals, the daily rhythm of pineal melatonin synthesis is controlled by neural inputs. The CNS is thought to be a primary target organ involved in mediating the influence of melatonin on a variety of physiological and behavioral processes, including biological rhythms, neuroendocrine function, activity levels and sleep. It now appears that melatonin is also produced in the retina and affects various aspects of retinal physiology. The purpose of this article is to provide a brief overview of potential regulatory sites involved in the production and action of melatonin. In particular, this review focuses on the rapid advances being made in the characterization and localization of melatonin receptors in the CNS, retina and pituitary and on recent findings pertaining to the regulation of melatonin synthesis in the mammalian pineal gland and retina.