Plasma levels of beta-thromboglobin (beta-TH) and of thromboxane B2 (TXB2) and 6-keto-prostaglandin F1-alpha, the stable metabolites of thromboxane A2 and prostacyclin, were determined by radioimmunoassay methods in eight patients with noninsulin-dependent diabetes before and after dietary treatment and after administration of the sulfonylurea drug glibenclamide. Blood examinations were performed when hyperglycemia was detected for the first time, four weeks after a dietary regimen was started, and four and eight weeks after glibenclamide treatment was begun. Drug treatment was instituted because, despite a suitable diet, patients' postprandial blood sugar was higher than 8 mmol/L (145 mg/dl). At the initial examination, elevated TXB2 and beta-TH levels indicating platelet hyperactivity and hyperglycemia were found. TXB2 and beta-TH levels decreased significantly after glibenclamide treatment was started, as did the blood glucose level. There was no change in 6-keto-PGF1-alpha. We interpret these results to indicate that diabetes is associated with hyperactivity of platelet aggregation and that control of blood glucose is important because a lower blood glucose level attenuates platelet hyperactivity. Whether the decrease in platelet hyperactivity is a direct result of the lowered blood sugar or reflects the influence of the drug treatment requires clarification.