When eight malnourished females without organic disease, were subjected to a controlled treadmill exercise test, they expended less total energy than females of normal body weight. The malnourished subjects consumed less oxygen than the control subjects, but oxygen consumption increased with increasing work load. The resting energy expenditure of the malnourished subjects was less than predicted values, but body composition as determined by muscle mass, total body water and thyroxine levels were within normal limits. Although decreased energy expenditure associated with malnutrition, has been attributed to decreased oxygen transport and altered hemoglobin, the malnourished subjects in this study did not have reduced hemoglobin levels. Metabolic adaptation may have occurred in order to improve the efficiency of aerobic metabolism. In order to confirm this theory, energy expenditure should be assessed under conditions of maximal oxygen intake.