Three experiments were performed to examine the effect of ingestion of medium- (MCT) and long-chain (LCT) triglyceride oils at the beginning of the normal feeding period on subsequent food intake of normal and diabetic rats. In the first experiment, diabetic rats reduced food intake more than normal animals in the first 6 hr after ingestion of 2.0 ml of MCT or LCT oil. In the second experiment, diabetic rats reduced food intake to a similar extent by 6 hr after ingestion of 1.5 ml of MCT or LCT oil, but the time course of this effect depended on the oil ingested. Ingestion of MCT oil produced a decrease in food intake within 2 hr, whereas ingestion of LCT oil reduced food intake 2-4 hr later. In the third experiment, a direct comparison was made of the differential time course of food intake suppression by MCT or LCT oil in both normal and diabetic rats. Diabetic rats decreased food intake after ingestion of 1.5 ml MCT or LCT oil, whereas normal rats did not. Again, in diabetic rats, ingestion of MCT oil produced a more rapid reduction in food intake than ingestion of LCT oil. It is proposed that the more pronounced reduction in food intake of diabetic rats after oil ingestion is due to a greater degree of hepatic oxidation of ingested fat, whereas the differential effect of MCT and LCT oil ingestion in diabetic rats is due to a differential rate of delivery of the ingested lipid substrate to the liver.