The purpose of this study was to find whether the addition of dietary lysine affected the rate of brain protein synthesis in aged rats fed on a gluten diet. Experiments were done on two groups of aged rats (30 wk) given the diets containing 5% gluten or 5% gluten + 0.3% lysine for 10 d. The fractional rates of protein synthesis in brain, liver, and kidney increased with an addition of dietary lysine. In brain, liver, and kidney, the RNA activity [g protein synthesized/(g RNA x d)] was significantly correlated 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 addition of the limiting amino acid for the low quality protein elevates the rate of protein synthesis in the brain of aged rats, and that RNA activity is at least partly related to the fractional rate of brain protein synthesis.