Studies in this report investigate the existence and quantitative contribution of a metabolic component to an apparently elevated efficiency of energy utilization during refeeding following low-calorie consumption. Energy balance during a 25-d refeeding period was assessed in rats that had been food restricted (at 50% of normal intake) for either 30 or 10 d. Relative to weight-matched controls (with comparable lean tissue mass and similar food intake), refeeding following both periods of low food consumption (30 or 10 d) was associated with a 10% reduction in energy expenditure (p less than 0.001). Analysis of body composition revealed that virtually all the energy saved as a result of this metabolic (as opposed to tissue mass) adaptation was deposited as fat rather than protein. After the energy cost for depositing the extra fat is accounted for, the metabolic component represents a net 15% lower energy expenditure when normal food intake is resumed. These adaptive changes in energy expenditure may thus constitute an important mechanism for the rapid replenishment of energy stores in preparation for recurring food shortages and may also underlie the ease with which the obese condition is rapidly reachieved after weight loss.