Cyclic 3-hydroxymelatonin (C3HOM) is an immediate product of melatonin's interaction with reactive oxygen species. Its presence has been detected in mice, rats and humans. In the current study, the antioxidant capacity and reducing power of this molecule have been systematically studied. C3HOM is found to be a more potent antioxidant than melatonin or vitamin C in terms of its ability to scavenge the hydroxyl radical (HO.) and to recover oxidized horseradish peroxidase to its ground state. The antioxidative mechanism of C3HOM is similar to that of the classic antioxidant, vitamin C, rather than to its precursor melatonin. C3HOM effectively prevents the oxidative degradation of cytochrome C induced by hydrogen peroxide (H2O2). It is speculated that some antioxidative activities of melatonin may be mediated by its metabolite, C3HOM. C3HOM prevents mitochondrial cytochrome C injury and, thus, it is likely to inhibit cellular apoptosis induced by the release of oxidized cytochrome C from mitochondria.