Cinnamaldehyde is an unsaturated aromatic aldehyde that is widely used as a food flavoring agent. The effect of cinnamaldehyde at the dose of 73.5 mg/kg body weight/day for 30 days, on male Wistar albino rats (80–90 g) was studied for an insight into the cellular antioxidant defense system. The antioxidant defense system in the liver of cinnamaldehyde-treated rats was different, as evidenced by the lower levels of glutathione and ascorbic acid. The primary antioxidant enzymes, superoxide dismutase and glutathione peroxidase activities were higher, whereas the catalase activity was diminished significantly. There was no appreciable difference in the activities of glutathione-S-transferase and glutathione reductase. The activity of glucose-6-phosphate dehydrogenase was significantly lower. Significantly higher levels of thiobarbituric acid reactive substances indicate oxidative damage. Thus, cinnamaldehyde treatment leads to the prooxidant state, which may be conductive for the carcinogenic process.