To determine whether platelet response to mental stress is altered, in essential hypertension, platelet aggregability and plasma beta-thromboglobulin were determined in 24 patients with essential hypertension (11 patients with World Health Organization (WHO) stage I and 13 patients with stage II) and 14 normotensive controls before and after a 10-min arithmetic stress (serial subtraction of 7 from 1000). In normotensive subjects, arithmetic stress did not affect primary aggregations to 1.0 mu mol/L adenosine diphosphate (ADP) and to 2.5 mu mol/L 5-hydroxytryptamine (5-HT), ADP threshold for biphasic aggregation and plasma beta-thromboglobulin level. In hypertensive patients with WHO stage I, these parameters were similar to those in normotensives before arithmetic stress, but the arithmetic stress test significantly increased primary aggregation to reagents and beta-thromboglobulin level, and decreased threshold of ADP for biphasic aggregation. In WHO stage II patients, platelet aggregability to reagents and beta-thromboglobulin level were already enhanced as compared with WHO stage I patients and normotensive subjects before arithmetic stress. However, the stress-induced changes in platelet function were less pronounced in WHO stage II patients compared with stage I patients. In conclusion, platelet aggregability and proaggregatory effect of mental stress differed depending on the severity of hypertension in patients with essential hypertension; the transient activation of platelet function during stress with no enhancement under the resting condition in the early phase of hypertension and the continuous activation of platelet function in the advanced phase with hypertensive organ damage. (C) 1999 American Journal of Hypertension, Ltd.