Increase in knowledge about reactive oxygen species action mechanisms and oxidative stress effects in living organisms led to intensive seeking for new, more effective substances, which prevent extreme development of oxidative stress or are able to decrease its negative influence, damaging cell structures and many cell functions. These substances are called antioxidants, scavengers, trappers or quenchers. In nineties, melatonin became the centre of the interest in the filed of investigation of antioxidative properties of different chemical substances. This is in living organisms ubiquitous substance with relatively simple chemical structure, good physical properties and wide physiological effects. The main role of endogenous melatonin comprises receptor-mediated biological rhythms synchronisation. Among other functions mentioned later belong anti-gonadotropic, immunotropic and non-receptor- mediated antioxidative effects. Melatonin is said to have also antineoplastic properties. Its anti-aging effect is discutable.

        Key words: melatonin, epiphysis, suprachiasmatic nucleus, antioxidation, hydroxyl radica l, lipoperoxidation, analgesic, opioid receptor.