Besides its actions via membrane receptors on cells in specific areas of the brain and peripheral tissues, melatonin also has direct intracellular actions. The neurohormone melatonin is a potent free radical scavenger in vitro and a powerful antioxidant in vivo. Studies to date indicate that it is a better scavenger of the highly toxic hydroxyl and peroxyl radicals than some other known compounds. Melatonin also prevents the toxicity of singlet oxygen and stimulates the antioxidative enzyme, glutathione peroxidase. Considering its varied and potent antioxidant capability, it is possible that melatonin is an essential element of the antioxidant defense system of organisms. Besides these direct intracellular actions of melatonin, it was recently discovered that the indole also has binding sites in the nucleus of many cells. Melatonin's genomic actions are believed to follow its binding to these nuclear loci.