The study population consisted of 51 healthy, growing, appropriate for gestational age LBW infants, all of whom were participants in an ongoing study of effects of quality of energy on rate and composition of weight gain in LBW infants. All studies were approved by Institutional Review Board and written consent was obtained from parents of all infants. This prospective feeding study is testing the general hypothesis that the quality of energy influences nitrogen utilization by LBW infants. In these studies infants are fed diets that provide a fixed protein intake of 4 g/kg per day, but differ in the percent of non-protein energy provided as carbohydrate and fat and in the gross energy content. The formulas are otherwise identical. As a part of these studies 6 h indirect calorimetric estimations of energy expenditure are performed every 2 weeks for each infant. During these 6-h periods continuous cardiorespiratory and behavioral state assessments are also made. Two studies were performed in each of the 51 infants for a total of 102 studies. All infants were in room air, without apnea of prematurity, receiving no respiratory stimulants or cardiotonic drugs, and had no sonographic evidence of central nervous system pathology at the time of the studies. The characteristics of the infants are shown in Table 1.