Melatonin (N-acetyl-5-methoxytryptamine) (MEL) is an indole-related neurohormone secreted mainly by the pineal gland. Although MEL freely diffuses across all biological membranes, their biological activity can be mediated via receptors. Spectrum of the membrane receptors of MEL includes MT1 (Mel1a), MT2 (Mel1b) and MT3 (Mel1c) (found in amphibians, birds and fish). Nuclear receptors MEL are also known, [RZR / RORa and NR1F2 (RZR / RORÃŸ)]. Melatonin is a direct and indirect scavenger of free radicals, which are highly toxic to aerobic organisms, such as the hydroxyl radical, hydrogen peroxide, singlet oxygen, nitric oxide (NO) and peroxynitrite anion; similarly, it reduces the accumulation of the main products of lipid peroxidation, and DNA adduct accumulation induced by carcinogens. In another context, the indirect function is fulfilled by promoting the expression of genes encoding enzymes with antioxidant functions or decreased expression of the genes for pro-oxidants, enzymes such as inducible nitric oxide synthase (iNOS). Pro-oxidant effects of MEL have not been described.