Dioscorea japonica Thunb its effects has been used in f31k remedies f9r various purposes including treatment of diabetes, on hypoglycemic actiity and energy metabolism were investigated. The plant was extracted with methanol(MeOH) and fractionated into four layers hexane, chloroform(CHCI$_3$), butanol(BuOH), and $H_2O$. Diabetes mellitus was induced in male Sprague-Dawley rats by the injection of streptozotocin(STZ) into tail vein at a dose of 45mg/kg body weight(BW). Sprague-Dawley male rats weighing 160-200g were divided into five groups a diabetic control and four experimental groups such as hexane group, CHCl$_3$ group, BuOH group, and $H_2O$ group. The rats of all groups were fed on a AIN-76 diet and the four experimental groups were orally administered each fraction(500mg/kg BW) for 12 days. The diabetic control group was orally administered 5% carboxymethyl cellulose. The body weights were monitored and the concentrations of blood glucose were determined. The levels of glycogen and protein in liver were also measured. The plasma levels of cholesterol, triglyceride (TG), and fee fatty acid(FFA) were also analysed. The body weight gain was higher in the $H_2O$ group than in the control group. Heart weight was significantly reduced by administrations of Dioscorea japonica Thunb. The extents of blood glucose decrement in BuOH and $H_2O$ group were greater than that found in the control group. The muscle protein levels showed significantly higher amounts in all experimental groups. Glycogen levels were higher in the BuOH group than in the control group. The levels of TG were decreased in all experimental groups and the levels of plasma FFA were lower in the BuOH group. The plasma cholesterol levels were not influenced by these four fractions in diabetic rats. These results suggest that the orally administered H2O fraction of Dioscorea japonica Thunb exhibited hypoglycemic effects in STZ induced diabetic rats. (Korean J Nutrition 31(7) : 1093-1099, 1998)