Energy balance is the resultant of ingested calories and energy expenditure and is generally maintained within narrow limits over prolonged periods. Exercise leads to an increase in energy expenditure which is, in the long-term, counteracted by increased energy intake. Evidence for this comes from a study in voluntarily running female rats that increased their daily food intake to 130% of the sedentary controls. In contrast, when considered on a short-term basis, exercise will suppress food intake to prevent a potentially dangerous disruption of energy substrate homeostasis. Studies in permanently cannulated rats submitted to a test meal and 2 hrs swimming reveal that both food intake and exercise lead to increases in glucose and free fatty acid (FFA) levels in the blood. These changes in glucose and FFA, combined with the exercise-induced alteration in among others glucagon, corticotropin releasing hormone (CRH) and body temperature, may lead to the short-term anorexic effect of exercise.