1. The effect of an energy-restricted (46 kJ day-1 kg-1), adequate protein diet (1.47 g day-1 kg-1) on the nitrogen metabolism of five obese rapidly growing adolescents (two males and three females) was assessed by means of nitrogen-balance measurements and determination of whole-body nitrogen turnover. 2. The energy-restricted diet had no significant effect on nitrogen balance (P more than 0.1) for the entire group when compared with the control dietary intake; however, significant (P less than 0.01) differences in nitrogen balance were noted among individuals at each dietary interval. 3. Mean values for whole-body nitrogen turnover, protein synthesis and breakdown for the control period were: 45.5 +/- 13.2 mg of nitrogen day-1 kg-1, and 5.72 +/- 1.96 and 5.74 +/- 1.92 g of protein day-1 kg-1 respectively. These values are 82% of those measured in infants. 4. Reducing the mean non-protein energy intake to 20 kJ day-1 kg-1 had no significant effect on whole-body nitrogen turnover, protein syn;thesis or protein breakdown. 5. The results are discussed in relation to the regulation of whole-body nitrogen metabolism, by dietary protein and energy intakes.