SUMMARY The relatire contribution of the renin-angiotensin system, adenocorticotrophic hormone (ACTH) and plasma electrolytes in the response of plasma aldosterone to 30 minutes of 65° bead-up tilt was assessed in 10 essential hypertensire patients. Studies were carried out before and during acute blockade of renin release by propranolol, ACTH suppression by dexamethasone and combined renin and ACTH blockade. In control studies orthostasis induced significant increases only in plasma renin activity and aldosterone. In contrast, when the renin response to tilt was acutely suppressed by propranolol administration, the aldosterone response was nonetheless maintained but now appeared to be under ACTH control, since concurrent increases In cortisol were observed. During ACTH suppression aldosterone increased during tilt and so did renin. However, during combined ACTH and renin blockade aldosterone failed to increase during tilt. These studies suggest that the aldosterone secretory response to head-up tilt is normally mediated by the renin-angiotensin system but, when the renin response is suppressed, an ACTH response is elidted which assumes a backup role. However, when these two systems are blocked other factors appear unable to respond during tilt to support a normal aldosterone response.