Resting metabolic rate is demonstrated to be a function of fat-free mass and a growth variable related to food-energy-input imbalance rate. By use of obligatory energy expenditure terms, the two-reservoir energy model applied to hyperphagia shows that growth of the fat-free mass is rapid whereas that of the fat store is slow and that the growth of both is bounded. Most of the excess energy at the onset of hyperphagia initially goes into the fat store, but this decreases with time until the greater fraction is diverted to the fat-free mass. The two-reservoir model predicts that weight gain per unit of excess energy is not constant but decreases monotonically until ultimately reaching an asymptotic value. Departure of theory and experiment in the long term suggests that facultative considerations become increasingly important.