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The growth of body protein, fat, and skeleton of growing steers was determined from published experimental data of steers fed three planes of nutrition. Each animal was partitioned into: 1) metabolically slow tissue which included hide, hooves, horns, skeleton, and fat, and 2) metabolically active tissue (MAT) which were the remaining tissues. MAT grew exponentially in time and approached the same final value for all planes of nutrition; however, growth rates depended on energy intake. The mass of active tissues was described by MATkg = 27 + 393 (1 - e-t/tau) where t and tau are in months; tau was 25.8, 46.7, and 57.4 months for the high, medium, and low planes of nutrition, respectively. The finding that MAT retained the same mathematical form but grew more slowly on the lower nutritional planes suggests that changes in energy set the scale of physiological age. Fat deposition was linear with time for all groups, although the rates depended on energy intake. The rates were 8.21, 2.08, and 1.29 kg/month for the high, mediu, and low planes of nutrition, respectively. The results suggest that steers are not programmed to synthesize protein first and then fat only with the remaining energy; rather the level of energy intake and the age determine how the ingested energy is partitioned into protein and fat synthesis.