The purpose of the study was to evaluate the possible effects of the administration of a green tea extract on the oxidative state of the liver and brain of adjuvant-induced arthritic rats, a model for human rheumatoid arthritis. Daily doses of 250 mg kg⁻¹ (59.8 mg catechins per kg) for 23 days were administered. This treatment produced significant diminutions in protein and lipid damage in liver, brain and plasma. It also diminished the tissue ROS contents and increased the antioxidant capacity of the plasma. The antioxidant defenses, which are diminished by arthritis, were improved by the green tea treatment, as revealed by the restoration of the GSH and protein thiol levels and by the strong tendency for normalizing the activities of the antioxidant enzymes. The activity of glucose 6-phosphate dehydrogenase, which is increased by arthritis in the liver, was also almost normalized by the treatment. In conclusion, it can be said that green tea consumption is possibly beneficial for the liver and brain of patients suffering from rheumatoid arthritis because it attenuates the pronounced oxidative stress that accompanies the disease and, thus, diminishes the injury to lipids and proteins in both liver and brain. There are also indications that, in the liver, the green tea can contribute to normalize the metabolic functions that are substantially modified by arthritis. For example, the green tea normalized the activity of glucose 6-phosphate dehydrogenase, a key enzyme of an important metabolic route (pentose monophosphate pathway). It is expected that the green tea treatment is equally able to normalize the activity of other enzymes (e.g., glucokinase and glucose 6-phosphatase), a hypothesis to be tested by future work.