To evaluate the effect of antihypertensive therapy on platelet activation in essential hypertension, the plasma levels of beta-thromboglobulin (beta-TG) were examined in 45 patients with essential hypertension and 20 age-matched normotensive control subjects. Hypertensive patients were assigned to monotherapy with one of five different antihypertensive drugs for 6 months, and the change of plasma levels of beta-TG was reexamined after the completion of the monotherapy. The plasma beta-TG increased in hypertensive patients compared with levels in normotensive control subjects. Monotherapy with each drug resulted in sufficient blood pressure control in all hypertensive patients. The plasma beta-TG decreased significantly after monotherapy with an alpha-blocker or an angiotensin-converting enzyme inhibitor (ACEI). The plasma beta-TG increased with the use of a diuretic but did not change with the use of a beta-blocker or calcium antagonist. The platelet activation observed in patients with essential hypertension is reversed by monotherapy with an alpha-blocker or an ACEI. It is possible that these drugs reduce the development of hypertensive vascular complications due to suppression of platelet activation in patients with essential hypertension.