Absorption and oxidation of medium-chain triglyceride (MCT) and long-chain triglyceride (LCT) infused in vivo into intestinal loops (Thiry-Vella loop) were studied. Thiry-Vella loops were formed in rat peritoneum using 20cm of jejunum resected from the same animal, both ends being attached open at the peritoneal wall. Thus each prepared loop was used as an in vivo intestine system free from bile and pancreatic juice. Normal absorbability in each loop was confirmed by infusion of a [¹⁴C] glucose. Three hours' cumulative absorption of [¹⁴C] MCT and [¹⁴C] LCT infused into the loop was assayed. The absorption rate of [¹⁴C] MCT was significantly higher than that of [¹⁴C] LCT, being 70.1±14.4% and 19.7±6.3%, respectively. Three hours' cumulative expired ¹⁴CO₂ was also measured in the same animal model as that for the fat oxidation assay. The ¹⁴CO₂ production from [¹⁴C] MCT was 18.8±6.1% of the infused dose, whereas that from [¹⁴C] LCT was only 1.7±1.1%. These results indicate that enterally administered MCT is a more effective energy source than LCT in the absence of bile and pancreatic juice.