The effects of dietary energy concentration on tissue composition and nitrogen metabolism were determined by comparative slaughter and nitrogen balance trials in Omani male lambs during the hot summer months (July to October; maximum temperature 48 °C). Weaned lambs (n = 10 per diet) were fed on one of three isonitrogenous (160 CP g/kg DM) diets that contained low (9·98 MJ/kg DM), medium (10·3 MJ/kg DM) and high (11·4 MJ/kg DM) metabolizable energy contents. An initial slaughter group of 10 animals was used to estimate the initial body composition. Treatment animals were slaughtered at 113–114 days. Increasing dietary energy concentrations resulted in a progressive increase in empty body, carcass and non-carcass water, protein and fat contents. Increasing dietary energy concentrations also resulted in a greater deposition of energy in carcass fat and a reduced deposition of energy in carcass protein. Dietary energy concentration did not affect the distribution of energy between protein and fat within empty body and non-carcass tissues. Ratios of energy to empty body, carcass or non-carcass weight were not affected by dietary energy concentrations and averaged 17·1, 18·2 and 15·9 MJ/kg respectively. These data indicate that more energy is required to deposit carcass tissues than non-carcass tissues. Nitrogen balance trials (feed N–faecal N–urinary N) conducted midway through the experiment indicated that dietary energy concentration had no effect on nitrogen digestibility or nitrogen retention. However, nitrogen retention determined by comparative slaughter showed that animals fed the low energy diet retained significantly less empty body nitrogen compared to animals fed the high energy diet. Thus, nitrogen retention determined by nitrogen balance trials overestimated direct measurements of nitrogen retention determined by comparative slaughter and this overestimation was greater on the low energy diet.