Plasma catecholamine concentration and platelet aggregation were studied in 22 patients with uncomplicated primary hypertension and 13 age-matched normotensive, healthy subjects at rest and in some during isometric handgrip exercise. The effect of norepinephrine (NE) infusion upon platelet aggregation was also examined. Plasma catecholamine concentration was slightly higher in the hypertensive than the normotensive group, but the difference was not significant. However, platelet aggregation to ADP was significantly greater in the hypertensive than the normotensive subjects. Exercise increased significantly both catecholamines and aggregation in both groups. Platelet aggregation was correlated with age (r = 0.62, P less than 0.01) and plasma NE (r = -0.34, P less than 0.05 for the total group of subjects). The infusion of NE increased significantly plasma NE and platelet aggregation and there was an inverse correlation between NE increase and threshold decrease (r = -0.69, P less than 0.05). Thus, plasma catecholamines and important determinants of platelet aggregation. However, in our study, uncomplicated primary hypertension was not associated with abnormal plasma catecholamine concentration. It is likely that the observed abnormal platelet aggregability to ADP represents a secondary phenomenon, possibly related to more advanced atherosclerotic vascular changes in hypertensive than normotensive subjects.