The present study was performed to determine the effect of long-term feeding with a high-fat/low-carbohydrate (HF/LC) diet on the onset of type-2 diabetes mellitus in normal rats. Male Wistar Imamichi rats were kept on a Control (carbohydrate 60%, fat 15%) or HF/ LC (carbohydrate 10%, fat 65%) diet for 16 mo. An intraperitoneal glucose tolerance test was performed once every 2 mo. Glucose tolerance was impaired 2 mo after the start of HF/LC diet feeding, accompanied by a decrease in the insulinogenic index. Along with time of HF/LC diet feeding, the glucose tolerance was further deteriorated with more serious impairment of insulin secretion and sensitivity. At the end of the experiment, 15 of 18 rats in the HF/LC group were diabetic, whereas only 4 of 17 rats in the Control group were diabetic. The present results demonstrate that longterm feeding with a HF/LC diet decreases the secretion and sensitivity of insulin, and induces diabetes mellitus in rats. Furthermore, long-term feeding with such a diet may produce adverse effects on the blood plasma lipid profile, with elevated levels of triglycerides, nonesterified fatty acids, total cholesterol, and reduced levels of high density protein cholesterol in the plasma.