It has been previously shown that a diet containing medium-chain triacylglycerols (MCT) leads to less body fat accumulation as compared to a diet containing longchain triacylglycerols (LCT). We investigated the involvement of diet-induced thermogenesis in the accumulation of body fat in rats fed a diet containing MCT. Twelve male Wistar rats were administered 1 g of MCT or LCT by gavage, and their oxygen consumption was measured for 6 h (experiment 1). Forty male Wistar rats were fed a diet containing 10% MCT or LCT for 6 wk, and their body composition was determined (experiment 2). In experiment 1, oxygen consumption increased to a greater extent after MCT administration than after LCT administration. Diet-induced thermogenesis was significantly (0.67±0.14 kcal) larger after the administration of 1 g of MCT. In experiment 2, there were no differences in food intake or carcass protein content between the LCT group and MCT group. However, carcass fat and intra-abdominal fat content were significantly lower in rats fed MCT than in those fed LCT. We calculated that ingestion of 1 g of MCT decreased body fat by 0.94±0.27 kcal relative to the ingestion of LCT. These results suggest that the larger diet-induced thermogenesis observed in rats fed MCT, compared to that of those fed LCT, is one of the main factors involved in the suppression of body fat accumulation in rats fed MCT.