Melatonin is the main secretory product of the pineal gland of all mammals, including humans. Functionally, melatonin has been linked to the strengthening of circadian rhythms, regula- 67 tion of seasonal reproduction and sleep processes. That melatonin functions as an antioxidant and affects cell proliferation and death was only uncovered in the last decade. The following article describes known facts about the role of melatonin as an antioxidant and its influence on cell proliferation and apoptosis. All these activities of melatonin indicate that it may be useful in the treatment of various conditions.