Sixty Dorper lambs, comprising 30 ewe and 30 ram lambs, were divided into six groups of five ewe and five ram lambs each. Each group received, on an ad libitum basis, one of six diets consisting of a concentrate: roughage ratio of 30:70, 40:60, 50:50, 60:40, 70:30 or 80:20. The lambs were weaned at 120 days when the mean weaning masses were 22,07 ± 0,66 kg for ewes and 22,13 ± 0,39 kg for rams. The trial started at weaning and ended when animals reached a body mass of 40 kg. Values for feed intake and body composition were determined during the active growth period. Body protein and body fat were found to be exponentially related to cumulative ME intake as the independent variable. Partial efficiencies for protein and fat deposition were calculated from these relationships. Differences due to gender were found between the partial efficiencies of protein and fat deposition at a particular body mass. The efficiencies of protein deposition of both sexes decreased with an increase in body mass, while those of fat deposition increased slightly. Consequently, the validity of the hypothesis that efficiencies of protein and fat synthesis are constant is questionable. According to the energetic efficiency (MJ/kg), the cost of fat deposition is higher than that of protein.