1. The effects of level of feeding of either a protein-adequate or a protein-deficiency diet on nitrogen retention (NR), growth performance, body composition and some aspects of energy utilization in pigs growing from 1.8 to 6.5 kg live weight (LW) were investigated in two experiments. 2. In Expts and 2 piglets were given a protein-adequate diet at four levels of intake (0.93, 1.44, 1.83 and 2.30 MJ gross energy (GE)/kg LW0.75 per d) and a protein-deficient diet at five levels of intake (1.14, 1.38, 1.68, 1.95, and 2.30 MJ GE/kg LW0.75 per d) respectively. 3. For pigs given the protein-adequate diet (Expt 1) NR was linearly (P less than 0.001) related to energy intake (EI) and independent of N intake (NI). NR in pigs given the protein-deficient diet (Expt 2) was linearly (P less than 0.001) related to NI and independent of EI. 4. Average daily LW gain responded linearly to increases in EI in both experiments. However, at equivalent levels of EI pigs given the protein-adequate diet exhibited more rapid and efficient growth than those given the protein-deficient diet. The results also indicated an interaction between the effects of EI and dietary protein content for feed conversion efficiency. 5. Body fat at 6.5 LW increased in a curvilinear fashion with increasing EI in both experiments. However, over the range of EI tested (from approximately 1.8 to 4.6 times energy for maintenance) body fat increased by 153% in Expt 1 and by only 27% in Expt 2. Pigs given the protein-deficient diet were also markedly fatter than those given the protein-adequate diet. Body protein at 6.5 kg LW decreased (P less than 0.01) with increasing EI in Expt 1 but was unaffected by EI in Expt 2. 6. As estimated by multiple regression analysis, the values for the deficiency of energy utilization for protein (kp) and fat (kt) deposition were 0.76 and 0.78 respectively in Expt 1 and 0.42 and 0.89 respectively in Expt 2. The estimates of metabolizable energy required for maintenance were 4.45 and 532 kJ/kg LW0.75 per d for Expts 1 and 2 respectively.