The thermic effects of 400 kcal meals of medium-chain triglycerides (MCT) and long-chain triglycerides (LCT) were compared in seven healthy men. Metabolic rate was measured before the meals and for 6 h after the meals by indirect calorimetry. Mean postprandial oxygen consumption was 12% higher than basal oxygen consumption after the MCT meal but was only 4% higher than the basal oxygen consumption after the LCT meal. There was a 25-fold increase in plasma beta-hydroxybutyrate concentration and a slight increase in serum insulin concentration after MCT ingestion but not after LCT ingestion. Plasma triglyceride concentrations increased 68% after the LCT meal and did not change after the MCT meal. These data raise the possibility that long-term substitution of MCT for LCT would produce weight loss if energy intake remained constant.