To assess whether and how the activation of the renin-angiotensin system that occurs in response to changes in posture contributes to the maintenance of blood pressure, we measured blood pressure, heart rate, plasma noradrenaline and plasma active and inactive renin in patients with essential hypertension in the supine, sitting and standing positions, (each sustained for 30 min), before and after administration of captopril. These studies were performed before and after a brief period of diuretic therapy. Both in the normal and in the sodium depleted state captopril reduced the normal adaptive response of blood pressure to changes in body position, whereas the responses of heart rate and plasma noradrenaline to these stimuli were unaffected by captopril. In contrast, the postural increases in active renin were magnified after captopril while inactive renin was unchanged throughout these acute studies. Our results indicate that during mild but rather prolonged postural stresses the generation of angiotensin by active renin supports blood pressure either through its direct vasoconstrictive effect and/or by potentiating the activity of the sympathetic nervous system. It is unlikely that the changes in active renin depend upon conversion from the inactive form.