The purpose of this study was to determine whether the quality of dietary protein affects the rate of brain protein synthesis in aged rats. Experiments were conducted on three groups of aged rats (30 wk) given diets containing 20 g casein, 20 g gluten or 20 g gelatin/100 g for 10 d. The fractional rates of protein synthesis in the brain, liver and kidney declined with the decrease in quality of dietary protein. In the brain, liver and kidney, the RNA activity [g protein synthesized/(g RNA.d)] correlated significantly with the fractional rate of protein synthesis. The RNA concentration (mg RNA/g protein) was not related to the fractional rate of protein synthesis in any organ. The results suggest that the rate of protein synthesis in the brain declines with the decrease in quality of dietary protein consumed by aged rats, and that RNA activity is at least partly related to the fractional rate of brain protein synthesis.