The protective effects of garlic (Allium sativum Linn.) on circulatory oxidant and antioxidant status during the initiation and post-initiation phases of carcinogenesis initiated with 4-nitroquinoline 1-oxide (4-NQO)-induced oral carcinogenesis were investigated in male Wistar rats. Animals were randomized into experimental and control groups and divided into five groups of six animals each. Animals in group 1-3 were given 20ppm 4-NQO in drinking water for 8 weeks. Group 1 received no other treatment. Group 2 animals were intragastrically administered aqueous garlic extract at a dose of 250 mg/kg body weight starting 1 week before 4-NQO exposure and continued until 1 week after the final exposure to the carcinogen. Group 3 animals received garlic extract (250 mg/kg body weight) starting 1 week after the cessation of 4-NQO treatment and continued until the end of the experiment. Group 4 animals received garlic extract alone. Group 5 animals received no treatment and served as the untreated control. The experiment was terminated at the end of 32 weeks. The extent of lipid peroxidation and the status of antioxidants were determined in blood samples from experimental and control animals. Enhanced lipid peroxidation in the circulation of tumor bearing animals was accompanied by a significant decrease in ascorbic acid, vitamin E, reduced glutathione (GSH) and glutathione peroxidase (GPx). Administration of aqueous garlic extract at a dose of 250mg/kg body weight significantly decreased lipid peroxidation and enhanced the levels of antioxidants both in the initiation and post-initiation phases. We speculate that garlic exerts its chemopreventive effects by modulating lipid peroxidation levels and enhancing antioxidant systems.