1. alpha-Glucosidase inhibitors delay carbohydrate absorption and have been proposed as adjunctive therapy for diabetes mellitus. 2. To determine the effects of two new alpha-glucosidase inhibitors, Bay-m-1099 and Bay-o-1248, on meal carbohydrate and lipid tolerance, plasma glucose, insulin and triglyceride levels were measured at 15-60 min intervals over 12 h after ingestion of a standard breakfast, lunch and dinner of identical composition in 31 normal volunteers. 3. The volunteers were randomized to receive either Bay-m-1099 (50 or 25 mg) or placebo prior to each meal, or the single administration of Bay-o-1248 (20 or 10 mg) or placebo prior to breakfast. 4. Only Bay-m-1099 at the 50 mg dose reduced significantly the postprandial increase in plasma insulin levels after each meal when compared with placebo (25, 36, 54% at breakfast, lunch, and dinner, respectively; P less than 0.05). Both drugs were well tolerated, with side effects limited to complaints of flatulence. 5. Thus, with the dosage schedule employed, Bay-m-1099, but not Bay-o-1248, significantly reduced postprandial increments in plasma insulin.