1. One hypothesis to account for altered adrenergic response in hypertensive patients is alteration in adrenergic receptors on responsive cells. We therefore used radioligand binding methods to examine the alpha 2-adrenergic receptors on platelets isolated from 17 normal men and from 19 men with essential hypertension. In these studies we used the alpha 2-selective radioligand [3H]yohimbine to determine receptor number and affinity on intact platelets. 2. The median number of receptors per platelet was 265 for the hypertensive patients versus 246 for the platelets of controls. Likewise there was no difference between hypertensives and controls in the dissociation constant of the receptors for [3H]yohimbine or adrenaline. 3. Anti-hypertensive treatment with the alpha 2-agonist guanabenz or the beta-antagonist propranolol did not change the number or affinity of platelet alpha 2-receptors. 4. In untreated hypertensives the receptor number did not correlate with age, blood pressure, or 24 h urinary excretion of catecholamines or Na+. 5. We conclude that neither hypertension nor anti-hypertensive treatment alters the number of alpha 2-adrenergic receptors on human platelets. Furthermore, because therapy with an alpha 2-agonist does not alter the receptor number and because the receptor number did not correlate with urinary excretion of catecholamines (an index of sympathetic-nervous-system activity), we conclude that 'down-regulation' of human platelet alpha 2-adrenergic receptors may not readily occur in vivo.