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Reactive oxygen species Melatonin was discovered to be a direct free radical scav-enger less than 10 years ago. Besides its ability to direct-ly neutralize a number of free radicals and reactive oxy-gen and nitrogen species, it stimulates several antioxida-tive enzymes which increase its efficiency as an antioxi-dant. In terms of direct free radical scavenging, melato-nin interacts with the highly toxic hydroxyl radical with a rate constant equivalent to that of other highly efficient hydroxyl radical scavengers. Additionally, melatonin re-portedly neutralizes hydrogen peroxide, singlet oxygen, peroxynitrite anion, nitric oxide and hypochlorous acid. The following antioxidative enzymes are also stimulated by melatonin: superoxide dismutase, glutathione peroxi