Bay o 1248 is a potent alpha-glycosidase inhibitor that reduces postprandial hyperglycemia when administered p.o. with sucrose or maltose. The compound binds to and competitively inhibits the alpha-disaccharidases and is also readily absorbed across the intestinal mucosa. To evaluate its effect on the activity of disaccharidases and on metabolic control, groups of obese diabetic mice (C57BLKsJ db/db) were given the drug for periods of 3, 7 and 84 days as a drug food mixture (5 or 10 mg/100 g of food). Nondiabetic mice of the same strain were dosed for 3 and 7 days. The drug did not influence body growth, food intake or fasting blood glucose. However, urine glucose excretion was significantly decreased at the higher dose in the diabetic mice. The drug had no effect on the protein content of jejunum (proximal and middle thirds) or ileum (distal third) of the small intestine. The activity of sucrase and maltase was significantly decreased in practically all segments of the small intestine in both diabetic and nondiabetic mice. These changes were evident after 3 days of drug administration. Lactase was not affected by the drug. The mechanism underlying these changes, although unclear, is of significant interest and deserves further investigation.