Whole body protein synthesis was investigated in growing male mice which were long-time selected for high carcass protein amount (DU-6P, protein line) or for high body weight (DU-6, growth line) and in the unselected randomly bred control (DU-Ks). Six mice/line were housed singly in metabolic cages for the estimation of N balance, whole body protein synthesis (end-product method, single dose of15N-labelled amino-acid mixture), and N distribution in the body. Another six mice/line were used for the determination of the body composition. All mice had free access to a commercial stock diet (crude protein 268 g, gross energy 19 MJ/kg dry matter) and to water. Body weight of both selection lines was about twice that of control mice at the same age. Selection for high body weight resulted in higher body fat content. Scaled to the corresponding body protein pools, the protein synthesis rates of selected mice were significantly higher than in controls, but were not significantly different between both selection lines in contrast to the protein deposition rates. The higher protein accretion in the protein line in comparison to the growth line seems to be due to a combination of a lower protein breakdown and an increased protein synthesis rate.