Antioxidant enzymes, including catalase, form the first line of defence against free radicals, therefore their regulation depends mainly upon the oxidant status of the cell. However, there are other factors involved in their regulation, including the enzyme-modulating action of various hormones such as growth hormone, prolactin and melatonin. Melatonin is a derivative of the amino acid tryptophan that acts as a neurohormone in mammals, but is also synthesized by many other species, including plants, algae and bacteria. Melatonin has been shown to markedly protect both membrane lipids and nuclear DNA from oxidative damage. Melatonin can directly neutralise several ROS, including hydrogen peroxide. It can also stimulate various antioxidant enzymes, including catalase, either by increasing their activity or by stimulating gene expression for these enzymes. The decrease in melatonin levels observed with age correlates with an increase in neurogenerative disorders such as Parkinson’s disease, Alzheimer’s disease, Huntington’s disease and stroke, all of which may involve oxidative stress. In general, the production of ROS increases with aging and is associated with DNA damage to the tissues.