Whole body protein turnover and tissue traits were studied in growing male mice long-term selected (78 generations) for carcass protein mass (DU-6P, protein line), for body weight (DU-6, growth line) or for an index combining body weight and treadmill performance (DU-6+LB, growth+fitness line) and in the unselected randomly bred control (DU-Ks). For the estimation of protein turnover data six mice of each line were housed individually in metabolic cages and were fed ad libitum a commercial stock diet (crude protein 268 g, gross energy 19 MJ/kg DM). At the end of the experiment body weight of all selected mice was about twice that of the controls. Fractional rates of protein synthesis (scaled to the corresponding body protein pools) were significantly higher in selected mice than in controls, but were not significantly different between the protein and growth line in contrast to the protein deposition rates. Chemical body composition was changed in dependence on selection traits. In the growth line body fat content was highest at the expense of protein content; in the protein line the proportion of lean body mass (muscle protein) was highest. This is supported by results of a previous experiment (generation 68) on 60-days old mice showing an enlargement of cross sectional area of the EDL muscle. The number of capillaries per fibre as indicator for nutrient and oxygen supply of the muscle w as adequately increased with the enlargement of muscle cross section in all selection lines, but it was even doubled in the growth+fitness line. No significant alterations in blood thyroid hormone levels (T₃, T₄ ) or in free glutamine content of muscles extracts were observed in response to selection. Plasma IGF-I levels were higher in selected lines than in controls.