Melatonin, N-acetyl-5-methoxytryptamine, the major hormone produced by the pineal gland under the influence of the dark/light cycle, has been shown to have a large number of therapeutic possibilities. It has been utilized in several countries for circadian rhythm disorders, sleep disturbances, jet lag, and sleep-wake cycle disturbances in blind people, and shift workers. In our mechanism of act, the G(i) protein-coupled metabotropic melatonin receptors MT1 and MT2 are the primary mediators of the physiological actions of melatonin. This hormone plays an important role in the regulation of physiological and neuroendocrine functions, such as synchronization of seasonal reproductive rhythms and entrainment of circadian cycles. In addition to its chronobiological role, several pharmacological effects of melatonin have been reported in mammals including sedative, antioxidant, anxiolytic, antidepressant, anticonvulsant, and analgesic activities. There is some evidence from clinical trials that melatonin can be helpful in that event. Current trends of pharmacological functions of melatonin pointed out its use in the treatment of neurodegenerative and neoplastic diseases. These effects and uses of melatonin are mentioned but further confirmatory studies are needed in most of them.