The possible protective effect of Melatonin was investigated for its antioxidant and lipid peroxidation activity against 7,12-dimethylbenz(a)anthracene (DMBA) induced mammary carcinogenesis in female albino Wistar rats. Mammary tumor was developed to the animals by administering 5mg/kg body weight of DMBA orally at weekly intervals for one month. Intraperitoneal administration of melatonin 5mg/ml per animals for 15 days prior to the first oral administration of DMBA was continued for a month. After the experimental period, oxidative stress parameters were assessed in plasma of both control and experimental groups. A significant increase in lipid peroxidation levels were observed in cancer induced rats while the activities of enzymic and non-enzymic antioxidants were decreased in cancer-bearing animals when compared to control animals. Administration of melatonin remarkably reduced the lipid peroxidation activity and increased the antioxidants level in drug treated animals. This result suggests that melatonin shows antioxidant activity and play a protective role against 7, 12-dimethylbenz(a)anthracene induced breast cancer. The inhibitory effect of melatonin on tumor cells and its lack of sideâ€“effects, indicate that melatonin should be considered as an adjuvant drug in the treatment of neoplastic diseases.