1. Aortic compliance and plasma catecholamine levels were determined in 26 essential hypertensive patients: 14 < 45 years (group 1) and 12 > 45 years (group 2). 2. Aortic rigidity, the reciprocal of aortic compliance, was calculated by the ratio of pulse pressure to stroke volume. Pulse pressure was derived by arterial pressure measured in the ascending aorta. Stroke volume was obtained by dye dilution. 3. A significant correlation (r = 0.60) was found between aortic rigidity and plasma noradrenaline in group 2. In this group both aortic rigidity and plasma noradrenaline levels were consistently higher than in group 1. 4. In 15 patients (seven of group 1 and eight of group 2) aortic rigidity was again measured after acute phentolamine (five patients) and labetalol (ten patients) administration. 5. Aortic rigidity was reduced significantly after both alpha-adrenoreceptor blockade alone and combined alpha- and beta-adrenoreceptor blockade. A highly significant correlation was found between aortic rigidity reduction and pretreatment noradrenaline levels (r = 0.77). 6. Aortic compliance is influenced, at least in part, by sympathetic nervous system activity. Drugs which block the peripheral effects of catecholamines may improve aortic compliance and consequently the control of systolic hypertension in selected cases.