The effects of gamma-aminobutyric acid (GABA) in rats with experimental diabetes mellitus were examined. Diabetes mellitus was induced in adult male Wistar rats by streptozotocin (STZ) injection. Oral administration of GABA (100 or 200 mg/kg body weight/d) for 10 d to the diabetic rats resulted in a significant decrease in their serum glucose level. GABA also reduced the level of glycosylated protein in serum, indicating an improvement of hyperglycemic conditions. Rats with STZ-induced diabetes showed arrested body weight gain and an increase in both liver and kidney weight, whereas oral administration of GABA attenuated the organ hypertrophy induced by hyperglycemia. In addition, the degree of serum thiobarbituric acid (TBA)-reactive substance level was significantly lower in the rats treated with 100 mg GABA, and the degree of TBA-reactive substance in the liver and kidney was reduced by GABA in a dose-dependent manner. These results suggest that GABA treatment protects against the development of diabetic complications resulting from impaired glucose metabolism and enhanced oxidative stress.