The total energy expenditure of the living organism has been the subject of interest and research since the eighteenth century when Lavoisier made the fundamental observation that energy is produced in the body by the process of oxidation. Kern, 1928;Bruin, 1931)). Some authors, Received January 31, 1966. therefore, prefer the term 'standard metabolism' (Bierring, 1931;Du Bois, 1936;Karlberg, 1952).With a baby, the fasting state is unsatisfactory for examining 'basal' metabolism. He is always digesting one meal or actively anticipating the next. Lee and Iliff (1956) showed in older babies and children that metabolic rate in the awake and fasting state is higher than when the child is in post-prandial sleep. Some workers have chosen the fasting state using a small dose of barbiturate to prevent 'uneasiness' (Baer, 1929;Schadow, 1929;Karlberg, 1952). The 'specific dynamic action' of food in infants is less pronounced than in adults, the maximal increase in energy metabolism being of the order of 4-10% after an ordinary feed (McCance and Strangeways, 1954;Levine, Wilson, and Gottschall, 1928).Until about 1940, the importance of environmental temperature and neutral range in babies was not fully appreciated. In 1943 Day, Curtis, and Kelly showed that premature babies more than a week old increased metabolism in the cold, but as recently as 1958 Silverman, Fertig, and Berger were unable to find satisfactory evidence that premature babies less than a week old had a metabolic increase in the cold. The result is that, in much of the early work, it is far from clear whether babies were in a neutral thermal environment when 'basal metabolism' was measured. To avoid assumptions implicit in terms 'basal' or