Medium-chain triglycerides (MCT) induce ketosis in several mammalian species including man. To clarify the regulation of this metabolic alteration, we fed rats either MCT or long-chain triglyceride (corn oil) and then attempted to correlate ketosis with changes in (i) concentrations of selected metabolites in plasma and (ii) the synthetic and oxidative capacities of the liver. By 1 hour after MCT feeding, plasma levels of total ketone bodies had increased 18-fold, with a maximum value reached 1 hour later. By contrast, total plasma ketones in rats fed corn oil were increased only about 2-fold at 2 hours after feeding and did not exceed this value at later intervals. Hepatic concentrations of ketone bodies also increased after MCT or corn oil feeding. Although plasma concentrations of glucose decreased and insulin increased in rats fed MCT, they were not affected by corn oil feeding. MCT-induced ketosis was depressed by glucose administration. Neither MCT nor corn oil feeding impaired utilization of glucose by the liver. Hepatic lipogenesis was suppressed 50% and 90% by MCT and corn oil feeding, respectively. A marked increase of long-chain fatty acids in plasma was observed in rats fed corn oil but not in rats fed MCT. The pronounced increase of ketones in MCT-fed rats was closely related to an elevation of octanoate. In liver slices of MCT-fed rats, ketogenesis from octanoate was 10-fold higher than from palmitate, and octanoate was oxidized 4 times more rapidly than palmitate. The ketosis of MCT-fed rats was depressed by administration of 4-pentenoic acid, a potent inhibitor of fatty acid oxidation. These results support the concept that ketosis induced by MCT stems from rapid oxidation of medium-chain fatty acids. Hyperinsulinemia, hypoglycemia and depressed lipogenesis resulting from MCT feeding appear to potentiate but not initiate ketosis.