The suitability of energy substrates used by the remnant liver after a 70% partial hepatectomy was studied in relation to the hepatic energy status in diabetic rats. Rats with streptozotocin-induced diabetes mellitus underwent 70% hepatectomy, and were divided into three groups receiving total parenteral nutrition (TPN) for 24h. One group received standard TPN without fat, while two other groups received standard TPN with medium-chain triglycerides (MCTs) or long-chain triglycerides (LCTs) as a 10% lipid emulsion. All rats of the TPN group without fat died within 24h. Blood ketone body ratios (aceto-acetate/beta-hydroxybutyrate), energy charge levels of the remnant liver, and cumulative amounts of 14CO2 in the expired breath during the 6h following 14C-labeled fat emulsion administration (MCT or LCT) were all significantly higher in the MCT group than in the LCT group 24h after hepatectomy. These findings suggest that medium-chain fatty acids (MCFAs) as an energy substrate are superior to long-chain fatty acids (LCFAs) during the immediate posthepatectomy phase in diabetics.