Animals, prevented from growing and then refed a good diet ad libitum, growth faster than normal and this is called "catch up" growth. This experiment was designed to show whether this is achieved by eating more food or by a greater efficiency of energy utilisation. Groups of rats (either 3 or 5 weeks old) were maintained at constant weight for a month, either by reducing the quantity of a good diet or by reducing the proportion of dietary protein. Food intake and carcass gain were measured for a period of 6 days "catch up" growth and compared with the control growth of rats with the same starting weight. It is concluded that "catch up" growth is associated with increased energy intake and increased efficiency when expressed as food conversion and gross energetic efficiency. There are, however, specific age differences in the metabolism of "catch up" growth which may be due to variation in either the maintenance requirement or the efficiency of protein and fat synthesis. This experiment emphasizes the differences between the various available expressions of energy utilisation.