The calorigenic effect of an artificial formula, Adapta, was examined in premature infants kept a t and below the neutral temperature. After a preliminary period of 15-30 minutes, a feeding (40-60 ml) was offered or given by gavage. The test feeding was followed by an experimental period of four hours, during which time oxygen consumption and CO, production were continuously recorded by an open circuit method using the Kipp diaferometer.The results of investigations carried out in the zone of thermoneutrality are summarized in table I. The amount of oxygen consumed 30 minutes after ingestion of the formula was already appreciably larger than that corresponding to the basal metabolic rate. At 90 or 120 minutes, the average oxygen consumption was 30 % higher than that a t the preliminary level. Oxygen consumption then fell and had approached the fasting level by the end of the period of observation.I n order to obtain the cumulative effect of thermogenesis, total and basal heat production were calculated for a period of four hours. The average amount of total extra calories was 1.7 kcallkg which, in relation to basal heat production during the same period of time and to the ingested calories, represented an increase of 26.4 and 9.2%, respectively (table VI).The changes in oxygen consumption a t different levels of thermoregulatory heat production in totally and partially swaddled premature infants maintained at 20-22' are shown in tables 11, I11 and IV. The responses are also diagrammatically compared (fig. 4). I t can be seen that the magnitude of specific dynamic action is the same for babies in a thermoneutral and colder environment, despite the difference in the preingestion levels of oxygen consumption. Control experiments in nonfed premature infants revealed that the specific dynamic action of food in a heat-losing environment is not due to muscular activity, but appears in an additive manner and independently of the metabolic increase induced by the cold environment (table V).The summation of the metabolic response to cold and to food was also reflected by the behavior of body temperature. The ingestion of 50 ml of formula stopped the fall in body temperature and even caused a small transient rise during the period of maximum increase in heat production.