Elevated silicon levels have been found in the serum of uremic patients, in the brain of patients with senile dementia and in neuroglial tangles of Alzheimer patients. The effect of silicon on superoxide dismutase was studied in vitro, since excessive superoxide production occurs in renal failure, in inflammatory conditions and in the aging process. Silicon in concentrations similar to those found in serum of uremic patients inhibits superoxide dismutase activity. The degree of inhibition is directly proportional to silicon levels. Depression of superoxide dismutase by Si is likely to result in a decrease in oxygen free radical destruction and thus an increase in excessive local availability of oxygen free radicals. The increased silicon levels in brain, kidney, lung and RBC which are especially sensitive to oxygenation damage may contribute to a variety of important clinical complications, by means of excess oxygen free radicals.