1. Adrenocorticotropic hormone (ACTH; 10 i.u./day) was infused for 34 h into normal male subjects. Some subjects were additionally treated with propranolol or indomethacin. Others received sham infusions or hydrocortisone infusions instead of ACTH.

2. ACTH, but not sham or hydrocortisone infusions, led to a significant increase in plasma renin activity and angiotensin II concentration with a lag period of 7–10 h and a maximum response after 24 h. ACTH may be a physiological regulator of renin secretion, perhaps through a ‘trophic’ effect on the juxtaglomerular apparatus.

3. The effect of ACTH on renin is not mediated by a rise in plasma renin substrate, probably not by renal β-adrenoreceptors, but perhaps by prostaglandins.

4. A dissociation between plasma cortisol and aldosterone during ACTH infusion suggests that ACTH, in this dosage, stimulates aldosterone on the second day through renin and angiotensin II, before its secretion is finally suppressed during more prolonged infusion.

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