1. Broiler chickens were fed 60-100% of recommended energy intakes to study the effects of energy restriction on protein and fat retention. 2. At an energy retention of 179 kJ/kg W0.75 d, only protein was retained. At higher energy intakes, each increment in retention had a rather constant composition: about 85% energy in fat and 15% in protein. At lower energy intakes body fat was mobilised whereas protein was deposited. 3. The efficiencies of energy retention in protein and fat were estimated to be 0.66 and 0.86 respectively. 4. The rather constant composition of additional retained energy after additional energy supply provides an explanation for a linear relationship between energy intake and energy retention.