Basal platelet function was measured in 35 40-year-old men with untreated mild essential hypertension and compared with 44 age-matched normotensive men. The groups differed significantly with respect to platelet size in venous blood (hypertensive, 7.46 +/- 0.10 X 10(-15) l versus normotensive, 7.11 +/- 0.09 X 10(-15) l; P = 0.01) and arterial concentration of the platelet-specific protein beta-thromboglobulin (hypertensive, 1.11 +/- 0.23 nmol/l versus normotensive, 0.59 +/- 0.04 nmol/l; P = 0.02). The normotensive subjects had significantly higher beta-thromboglobulin (BTG) in venous than in arterial blood (P less than 0.01). The hypertensive men showed no such difference. In contrast to the normotensive subjects, the hypertensive group had reduced arterial compared with venous platelet count (P less than 0.01). This may reflect an increased liability in the hypertensive subjects to lose platelets through adherence to the cannula during arterial blood sampling. The above findings point to increased platelet activity in essential hypertension, particularly in arterial blood.