No information is available about the effects of Japanese radish sprout (JRS) on diabetes. To clarify the effects, the influence of JRS on carbohydrate and lipid metabolisms was investigated in normal and streptozotocin-induced diabetic rats. The rats were fed a diet containing 0%, 2.5% or 5% of JRS ad libitum for 21 days. Compared with the corresponding control groups, the JRS-fed normal rats showed lower plasma levels of total cholesterol (TC), triglycerides (TG), phospholipids (PL), fructosamine, glucose and insulin and higher plasma levels of low-density lipoprotein-cholesterol, whereas the JRS-fed diabetic rats showed lower plasma levels of fructosamine, glucose and insulin without changes in the plasma lipid parameters. JRS also decreased the hepatic TC, TG and PL levels in the normal rats and the TG level in the diabetic rats. These results showed that JRS had a hypoglycemic activity in both the normal and diabetic rats and partly improved lipid metabolism in the normal rats. JRS has the potential to alleviate hyperglycemia in cases where diabetes is present and to serve in the primary prevention of diabetes mellitus. Copyright © 2006 John Wiley & Sons, Ltd.