1. Indirect calorimetry and primed constant infusion of [U-13C]glucose were combined in 28 appropriate-for-gestational age newborn, parenterally fed infants, in order to measure glucose utilization and glucose oxidation and to estimate lipogenesis from glucose. 2. The infants were randomly allocated to either a group receiving glucose as the non-protein energy source or a group having one-quarter of the glucose energy replaced by intravenous fat. The energy intake (370 kJ day-1 kg-1) and protein intake (3.4 g day-1 kg-1) were similar in both groups. 3. Energy expenditure (P less than 0.005), non-protein carbon dioxide production (P less than 0.005) and non-protein oxygen consumption (P less than 0.05) were lower in the lipid-supplemented group. 4. The significant excess of glucose utilization over oxidation (P less than 0.001) can be accounted for by lipid synthesis from glucose. 5. Fat synthesis from glucose was higher in the glucose/amino acid group (P less than 0.02), but total fat storage was higher in the lipid-supplemented group (P less than 0.02). Nitrogen balance was similar in both groups. 6. As lipogenesis from glucose is an energy- and oxygen-consuming and a carbon dioxide-producing process, the data suggest that the differences between the glucose-only group and the lipid-supplemented group are due to different rates of lipogenesis from glucose.