The energy metabolism of healthy infants, from 1 through 37 months of age, was measured with an open circuit chamber within 3½ hours of the time they had been fed and while they slept with little or no motion. Satisfactory results were obtained in 248 tests on 38 boys and 246 tests on 40 girls. A statistical analysis of the results of these determinations included the means, the standard error of the means, and the standard deviations of the means for cal/hr, cal/hr/m2, cal/hr/kg, and cal/hr/cm referred to age, as well as cal/hr referred to weight, height, and surface area, respectively, for boys and girls. Scatter diagrams for cal/hr and cab/hr/ m2 referred to age for boys were given. In order to differentiate normal thyroid activity from abnormal activity in children of this age, it was found that the best methods of reference to use for comparison, i.e., the methods which gave the least dispersion from the mean in healthy infants, were cal/hr/m2 referred to age and cal/hr referred to surface area, while the least desirable methods of reference for such use, i.e., the methods which showed the widest dispersion from the mean, were cal/hr/kg and cal/hr referred to age. Tentative standard values obtained from the mean trend lines were given for cal/hr/m2 for age and cal/hr for surface area. It was found that, from ½ to 3½ hours after feeding, there was no change in the effect of food on the metabolic rate or on the respiratory quotients. No significant changes were found in the respiratory quotients with age. It was concluded that differences between the energy production of boys and girls up to 3 years of age could be attributed to differences between the sexes in body size for age and body weight for height. The mean heat production of children from 18 to 36 months of age was found to be from 3 to 9% lower in tests done during sleep in the postprandial state than in those done during the usual awake fasting conditions.