The objective of this trial was to quantify the influence of a varied dietary crude protein content supplying equal amounts of limiting essential amino acids on parameters of energy metabolism. A total of 16 castrated male pigs were allocated to four dietary treatments and subjected to measurements of N-balance and gas exchange at approximately 65 and 85 kg live weight. Two ratios formulated to be isoenergetic differed in crude protein content (CP) and were offered at two feeding levels. At 85 kg, rations with a lower CP content were used. Multiple regression analysis was used to determine the effects of CP and those of live weight and metabolizable energy intake (IME). A rise in CP significantly increased energy losses via urine and the energy retained as protein (RPE), while the energy retained as fat was significantly reduced. Energy retention tended (P < 0.1) to decrease, while related to IME the reduction was non significant. The proportion of RPE in energy gain was significantly improved. Possible mechanisms for this shift towards reduced fat and energy retention at higher protein intakes are discussed. It is concluded that in practical feeding, rations with a lower CP content should be fed restrictively or be reduced in energy content to avoid excessive carcass fatness.