Rats with atrophic intestinal mucosa due to enteral nutrition 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 administration of the 10% FED. Ornithine decarboxylase activity in the ileal mucosa increased 3 days after the administration of 10% FED. Rats with atrophic intestinal mucosa that had undergone a 70% proximal jejunoileectomy, received an oral ED containing 4% long chain triglycerides (4% FED). In the jejunoileectomized rats, marked proliferation of the remaining ileum was observed irrespective of diet, when compared with the transected control group. In the transected group, the 4% FED had trophic effects on the ileum, but in the jejunoileectomized group, the 4% FED had no significant 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.