In order to compare the metabolic and hemobiological properties of two sulfonylureas, thirty-five non-insulin-dependent diabetics were randomly assigned to two groups. Each group was given either gliclazide (n = 20) or glibenclamide (n = 15) for six months. Metabolic control was improved in both groups, as evidenced by the decrease in HbA1 concentrations. A significant fall in ADP (1 and 4 microM)--induced platelet aggregation was recorded in the gliclazide group, while antithrombin III levels remained normal throughout the trial and plasminogen activator levels increased. These hemobiologic changes may be effective in preventing the vascular complications of diabetes. In contrast, glibenclamide did not alter platelet aggregation. Furthermore, a significant decrease in both antithrombin III levels and basal and venostasis-stimulated plasminogen activator levels were seen in glibenclamide patients. The beneficial changes in hemostasis seen in gliclazide patients probably result from a direct effect of the drug since hemobiological parameters and metabolic control parameters were not correlated.