The effect of an insulin-induced hypoglycemia was examined in 14 type I diabetic patients. After an overnight blood glucose normalization, each patient received an additional intravenous bolus of 3 U regular insulin at 0900 h (time 0). Blood glucose was continuously recorded up to 180 min. Plasma samples were assayed for beta-thromboglobulin (beta TG, ng/ml), pancreatic glucagon (pg/ml), cortisol (microgram/dl), and growth hormone (ng/ml) 30 min before the insulin stress, at time 0, at blood glucose nadir, and at 180 min. The blood glucose fell from a baseline level of 85.0 +/- 3.2 mg/dl to a nadir value of 39.2 +/- 1.9 mg/dl (P less than 0.001) reached at an average time of 41.4 +/- 4.9 min. Plasma beta TG increased significantly (P less than 0.05) during the insulin stress: 93.4 +/- 23.7 ng/ml at nadir versus 42.5 +/- 5.9 at time 0. Plasma cortisol and growth hormone were significantly increased (P less than 0.02 and P less than 0.01) at nadir compared with time 0 values. Plasma pancreatic glucagon was higher at nadir than at time 0, but the difference was not significant. The present results indicate that in vivo platelet activation can be triggered by hypoglycemic episodes in insulin-treated diabetic patients.