SummaryRats with atrophic intestinal mucosa due to enteral nutri tion supplied by an elemental diet (ED) for 4 weeks or more, received a fat-enriched ED containing 10% long chain triglycerides (10% FED) orally. The atrophic ileal mucosa became trophic 4 weeks after adminis tration of the 10% FED. Ornithine decarboxylase activity in the ileal mucosa increased 3days after the administration of 10% FED. Rats with atrophic intestinal mucosa that had undergone a 70% proximal jejunoile ectomy, received an oral ED containing 4% long chain triglycerides (4% FED). In the jejunoileectomized rats, marked proliferation of the re maining ileum was observed irrespective of diet, when compared with the transected control group. In the transected group, the 4% FED had trophic efects on the ileum, but in the jejunoileectomized group, the 4% FED had no siginificant trophic effect on the remaining ileum. In conclusion, long chain triglycerides had mild trophic effects on ileal mucosa and were effective in the treatment of atrophic intestinal mucosa. However, the trophic effects of fat were apparently masked by the marked proliferation of the ileal mucosa following jejunoileectomy.