The effect of chronic administration of medium-chain triglycerides (MCT) on swimming endurance (swim capacity) was investigated in male Std ddY mice. The mice were fed a diet containing 80 g MCT + 20 g long-chain triglycerides (LCT)/kg diet for 6 wk; mice fed diet containing 100 g LCT/kg diet were used as controls. After being accustomed to swimming, the mice were subjected to forced swimming every 2 d in the current water pool that we had developed, and the total swimming period until exhaustion was measured. The total swimming period was used as the index of swim capacity. The group fed MCT showed significantly greater swim capacity than the control group (89.5 +/- 2.5 vs. 80.2 +/- 2.0 min). In another experiment, after 4 wk of MCT diet consumption, significantly greater swim capacity was found in untrained mice. The major metabolic consequences of the adaptations of muscle to prolonged MCT administration during endurance training were higher activities of 3-oxo acid CoA-transferase (P < 0.01), citrate synthase (P < 0.1) and malate dehydrogenase (P < 0.1). These findings suggest that increases in the enzyme activities of the tricarboxylic acid cycle and ketone body utilization associated with the chronic administration of an MCT-containing diet enhance swim capacity in mice.