Recent studies have demonstrated that cholesterol synthesis is increased two to threefold in the intestines of streptozotocin-induced diabetic rats. Cholesterol synthesis in tissues other than the intestines, including the liver, was not significantly altered by diabetes. In diabetic Chinese hamsters, cholesterol synthesis was increased 2.5-fold in both the small and large intestine. These observations are similar to our findings in diabetic rats and suggest that a stimulation of intestinal cholesterogenesis may be a uniform phenomenon in insulinopenic diabetes. In db/db mice, cholesterol synthesis was increased in both the liver and intestines but quantitatively the increase in hepatic cholesterogenesis was of much greater magnitude. Cholesterol feeding, which markedly inhibited hepatic cholesterol synthesis in both control and db/db mice, did not obliterate this difference in hepatic cholesterol synthesis. In ob/ob mice, the severity of the metabolic disturbances was less than that observed in db/db mice and no abnormalities in cholesterol synthesis were observed in animals ingesting a low cholesterol diet. However, in ob/ob mice fed a high cholesterol diet, hepatic cholesterol synthesis was increased. These observations suggest that in obese insulin resistant diabetic animals of milder severity, the abnormality in hepatic cholesterol synthesis manifests itself only when the animals are ingesting a high cholesterol diet. The results of this and previous studies suggest that in insulinopenic diabetes there is a stimulation of cholesterol synthesis that is localized to the intestines, whereas in obese, insulin-resistant diabetic animals, cholesterol synthesis is altered in the liver.