The most accurate measurement of energy expenditure (EE) in free-living individual subjects is by the doubly-labelled water technique. The minute-by-minute heart rate (fH) method, while not yet suitable for individuals, will give excellent measurements of EE in small groups of subjects as well as the pattern of EE while wearing the fH recorder. In adults, the few studies available indicate that chronic energy deficiency (CED) results in decreased energy expenditure. In pre-school children, CED results in decreased activity as a first line of defense against decreased rate of growth. In marginally malnourished school children, the lower levels of EE are the results of their smaller size. However, they are unable to cope with situations of increased activity above ordinary levels. The relative effort exerted by control and undernourished boys and girls 6-16 y of age averaged approximately 25% VO2 max during 12 h of school days. There was a -3% change with age but there were no sex or nutritional status effects on average % VO2 max. It is suggested that this relative constancy in % VO2max in such disparate groups may be indicative of some physiological regulation of activity levels.