Plasma beta-thromboglobulin (beta TG) and platelet factor 4 (PF4) were significantly higher in a group of 116 hypertensive men than in a normotensive group of 142 men. They increased with the stage of hypertension but the level did not correlate with the age of the subjects. Platelet aggregation was similar in the two groups and positively correlated with the age of the subjects in the normotensive group but not in the hypertensive group. A strong positive correlation was observed between the levels of plasma beta TG and PF4 and between platelet aggregation to ADP and that to epinephrine in both the hypertensive and normotensive groups. However, there was no correlation between the level of plasma beta TG or PF4 and platelet aggregation. Plasma antithrombin III was lower in the hypertensive group than in the normotensive group. These studies suggest that plasma levels of beta TG and PF4 are closely related to the stage of hypertension and are better indicators than aggregation of in vivo platelet activation in hypertensive subjects. Enhanced platelet activation may be involved in the acceleration of hypertensive arteriovascular damage and atherosclerosis.