Alloxan-diabetic rats were kept on an ordinary or 0.25% cholesterol supplemented diet for up to 12 months, and changes in the serum lipid and lipoprotein levels were examined. The diabetic rats showed IRI levels of 3-S IU/ml, which were 1/5-1/10 that of the normal rats, and about 10 times higher serum glucose levels than the normal rats. The diabetic rats consumed 1.5-2 fold amounts of diet but their body weights were almost constant. The diabetic rats showed marked hyperlipidemia, especially in the cholesterol and triglyceride levels. The phospholipid level also increased but for a lesser extent and the c/p ratio increased. The serum lipoprotein levels increased in the diabetic rats, 5-10 times in the ordinary diet fed groups and 10-20 times in the cholesterol diet fed groups. Chylomicron, VLDL, IDL and LDL increased but HDL decreased. These changes were more significant in the diabetic rats fed the cholesterol diet. The triglyceride levels in the lipoprotein fractions increased in the diabetic rats, but the increase in cholesterol was more significant and the relative composition ratios of triglyceride decreased. The composition ratios of phospholipids, however, remained almost constant in all the lipoprotein fractions. The increase in IDL, especially cholesterol in the IDL fraction, was remarkable in the diabetic rats. The responsiveness of the diabetic rat VLDL to lipoprotein lipase (release of free fatty acids) was less than a half of that of the normal rat VLDL. In conclusion, the diabetic rats showed an increase in triglyceride but more significant increase in cholesterol in the lipoprotein fractions. These changes in serum lipid and lipoprotein levels and compositions appeared as early as 3 months after the alloxan injection and lasted for 12 months without any progress in the disturbance.