Melatonin, a hormone secreted by the pineal gland, is known to have anti-mutagenic and oncostatic actions. This beneficial action of melatonin has been explained in terms of its ability to scavenge free radicals and augment the activities of anti-oxidant enzymes. In the present study, we evaluated the effect of melatonin on benzo(a)pyrene-induced two stage skin carcinogenesis in mice and observed that it can not only decrease the number of animals bearing papillomas but also the number of papillomas per animal both in the initiation and promotion stages of skin carcinogenesis. It was also found that melatonin treated animals have low levels of lipid peroxides and that it can also prevent the binding of BP or its metabolites to DNA. In view of these evidence, it can be suggested that melatonin may function as an endogenous anti-mutagenic and oncostatic agent.