In a double-blind quadruple cross-over study the effect of a new alpha-glucosidase inhibitor (BAY g 5421) on postprandial blood glucose, serum insulin, and serum triglyceride increases was tested in 24 male healthy volunteers. They received before a standardized breakfast 50, 100, or 200 mg of BAY g 5421 or a placebo per os. The dose-time-response relationships were calculated and the drug tolerance was assessed. There was a statistically significant inhibition of the postprandial increases of the blood glucose, serum insulin, and triglyceride values. Further analysis showed no dose-dependent effect of the drug on the blood glucose values, whereas the serum insulin and triglyceride values were affected in a dose-dependent fashion. The maximal inhibitory effect on the serum insulin levels occurred 69 min after breakfast and on the serum triglyceride levels 104 min after breakfast. One hundred and 200 mg of BAY g 5421 were equally inhibitory-effective on the serum insulin levels, whereas the highest dose used was markedly more effective on serum triglyceride values than lower doses. Based on these results, a dosage of 100--200 mg of BAY g 5421/meal is recommended for clinical trials in metabolic diseases.