The relative effects of medium chain (MCT) and long chain triglycerides (LCT) on intestinal morphology and functions were compared. Adult rats received intragastrically for 10 days an isoenergetic mixture containing either 50% MCT/50% LCT or 100% LCT. The other constituents of the diets were identical, and animals fed a standard diet orally were used as a reference group. Animals who were given the MCT/LCT diet showed a higher mucosal mass and protein content and increased villus length and crypt depth in the proximal part of the small intestine compared with the LCT and control diet groups. Administration of [3H] thymidine 12 hours before death resulted in a significant increase in the incorporation of the precursor into cellular DNA in the jejunum of rats given MCT. In rats given LCT as the only fat, the free fatty acid content of the microvillus membrane showed a 20 fold increase and at the same time there was a significant drop in the cholesterol content and in the cholesterol/protein ratio. Differences in the lipid composition of enterol diet or in the microvillus membrane did not effect adversely membrane bound hydrolase activities. These findings suggest that MCT in the diet confers advantages in addition to the provision of rapidly available energy.