The effect of aluminum ingestion on the lipid peroxidation in rat brain, lung, liver, spleen and kidney was examined. Aluminum hydroxide, 100 mg/kg body weight, was administered orally once a day for 7 d, and the amount of lipid peroxide (TBA value) and the activity of superoxide dismutase (SOD) were measured 24 h after the last administration. Lipid peroxide was increased in the brain, to 142% of the control. TBA values in the lung, liver, spleen and kidney were similar to those in the control group. Pretreatment of rat brain, lung, liver, spleen, and kidney homogenates with aluminum chloride in an ice-bath increased the TBA value in the brain significantly compared with that of the control. Examination of variation in SOD activity showed that the activity in the brain was decreased, while that in the kidney was increased, compared with those of the control. Activities of SOD in the lung, liver, and spleen were similar to those of the control. These results suggest that an increase in lipid peroxidation and a decrease in activity of SOD in the brain after oral administration of aluminum hydroxide constitute one of the factors for the mechanism of brain injury by aluminum.