The energy metabolism of 37 infants with growth failure and malnutrition was investigated, by use of the insensible loss of weight (IL) as the index of metabolic rate. The IL was measured 4 to 6 (standard IL) and 12 hours (basal IL) after a regular meal, while asleep. Twenty infants were tested repeatedly throughout the recovery period. Control values for the IL, obtained in 27 normal infants, agreed well with established norms for IL and for basal metabolic rate, calculated from oxygen consumption. The initial standard and basal IL of the infants with growth failure fell between the regression lines on height of normal controls representing ±3 SE., but were above normal when related to body weight. The basal IL remained essentially within this range throughout the recovery period. By contrast, the standard IL rose upon realimentation with a high caloric diet sufficient to cause weight gain and declined during periods of weight loss or no weight gain. This rise, representing postprandial heat production in excess of basal (EHP), was greater than normal (up to 96%) for a period following realimentation. Peak values for EHP showed a near linear correlation with the average daily protein intake. This correlation appeared to be induced by the supply of sufficient calories, since the EHP was in a low range on admission and upon recovery regardless of the average daily protein intake. EHP did not correlate with the protein content of the meal given prior to measurement of the standard IL; therefore, factors other than those generally implicated in the specific dynamic action of food seem to be operative.