Antiobesity and antidiabetic actions of the alpha-glucosidase inhibitor AO-128 were examined using genetically obese-diabetic rats, Wistar fatty. Ten-week-old, male fatty rats were kept on CE-2 diet containing 10 or 25 ppm of AO-128 for 4 weeks. The average drug intake was calculated to be 0.74 or 1.78 mg/kg/day from the average food intake, respectively. The intestinal maltase and sucrase activities were decreased by AO-128 in a dose-related fashion. Food intake of fatty rats treated with AO-128 was decreased throughout the experiment. This decrease in food intake could hardly be explained only by diarrhea which occurred for the first 5 days of the administration of AO-128. AO-128 normalized hyperglycemia and markedly reduced hypertriglyceridemia and hyperinsulinemia in fatty rats. In addition, AO-128 decreased body weight gain, food efficiency, epididymal adipose tissue weight, carcass weight, and body fat deposition. These findings indicate that AO-128 may be useful for treating human obesity and diabetes.