Arterial concentrations of glycerol, FFA, glucose, lactate and β‐hydroxybutyrate were serially measured during the first two hours after birth in normal fullterm infants in a thermo‐controlled environment. Blood gas tensions, acid‐base balance, pulmonary gas exchange, motor activity and heart rate were also determined: a detailed report of these data will be published separately. In 22 infants the glycerol concentrations showed a rapid immediate increase after birth wheras the rises in FFA concentrations were delayed until between 30 and 120 minutes, indicating a prompt increase in lipolysis and a suppression of lipid mobilization during the first half hour after birth. This suppression might be explained by a high rate of reesterification or oxidation of FFA within adipose tissue. The influence of environmental temperature (2 8.7‐3 4.8 oC) and degree of acidosis on the pattern of changes in FFA and glycerol were only marginal. No inhibition of lipolysis and lipid mobilization was shown in an infant who developed postnatal asphyxia.At 120 min after birth, when acidosis had been eliminated, an inverse correlation was found between the rise in FFA from birth to 120 min and the respiratory exchange ratio (V̇co2/V̇o2).The glucose concentrations were related neither to the FFA nor to the glycerol concentrations. The rate of elimination of lactate and β‐hydroxybutyrate was not influenced by environmental temperature or acidosis. Minute amounts of administered heparin caused an increased rise in FFA and glycerol concentrations which were associated with the appearance of lipoprotein lipase activity.