A model for changes in energy balance and body weight is described which can be written as an iterative computer program. Medium and long-term changes in body weight resulting from random daily fluctuations in energy balance are quantitatively predicted. Body weight varies periodically about a mean value to to the extent of +/- 1 kg over a span of a few weeks. Long-term weight stabilization is the result of a dynamic equilibrium between energy intake and output which depends on activity and tissue metabolism. A lean : fat tissue deposition ratio defines the metabolic type of individual. This ration, which is constant for each individual, governs the proportions in which these tissues are both deposited and mobilized. In the model, the ratio has an important effect on the magnitude and composition of the weight gains resulting from over-eating. It is suggested that the differences in response which have been observed in over-eating experiments result from inter-individual differences in this ratio.