We have investigated the effect of inhaled oxygen tension on lipid metabolism during propofol infusion. Propofol is supplied as a lipid emulsion containing 10% soybean oil, which is rich in triglycerides (TG). Infused TG are metabolized via three pathways in the liver cell; Krebs cycle, ketogenesis and release as very low density lipoproteins (VLDL) into the blood. For this reason, we measured TG and the products of the three pathways; carbon dioxide, ketone bodies and VLDL. Thirty-two rabbits were anaesthetized under four different conditions: propofol under hyperoxia, normoxia, hypoxia and isoflurane anaesthesia under hyperoxia. Our results indicated that hyperoxia produced more ketone bodies, normoxia more PaCO2 and hypoxia more free fatty acids (FFA) and TG compared with the other propofol infusion groups. We conclude that hyperoxia during propofol infusion facilitated fat metabolism through ketogenesis, while normoxia did so via the Krebs cycle. Also, hypoxia suppressed utilization of TG and VLDL production in the liver.