This preliminary study was undertaken to investigate the effect of dietary silicon and aluminum on levels of these elements in brain. Two ages of rats, 22 day and 10 month, were assigned to 1 of 4 diets: 1) low silicon; 2) low silicon plus aluminum; 3) silicon supplemented; and 4) silicon supplemented plus aluminum. Rats were 23 and 28 months old upon termination of the experiment. Twelve brain regions were analyzed for silicon and aluminum. Regional variations in silicon, which were independent of dietary silicon supplementation, suggest that silicon may be an essential element in brain. Aluminum supplementation decreased the silicon content in selected brain regions, including those thought to be involved in Alzheimer disease. A relationship has been established between silicon, aluminum and age. In 23-month rats, aluminum supplementation did not increase brain aluminum content. By contrast, in 28-month rats, aluminum supplementation of the low silicon diet increased brain aluminum content in most regions. No increase occurred in silicon supplemented groups of the same age. Dietary silicon supplementation thus appeared to be protective against aluminum accumulation in aging brain.