1. Five experiments, each with three steers, were made in which the same diet was given in three different amounts and energy and N metabolism measured. Fasting metabolism was also determined in each experiment. The experiments were made when the steers were 15, 31, 35, 46 and 81 weeks of age.
2. Age of animal had no effect on the apparent digestibility of the diet, methane production and urine energy loss from or on the metabolizable energy of the diet. The efficiency with which the metabolizable energy of the diet was used to maintain the animals and to promote energy retention by them was also unaffected by their age. The determined efficiencies were those to be expected from the results obtained in experiments with mature ruminants.
3. Retention of N was more efficient in the younger animals, and the composition of the gains showed that protein energy accounted for 30.5% of the total energy retained in the 15-week-old animals and 24.8 % in the animals 81 weeks of age.