In three patients with diabetes and hyporeninemic hypoaldosteronism changes in renin activity, plasma aldosterone and cortisol were examined under various conditions: orthostasis and intravenous furosemide, infusion of synthetic beta1-24 ACTH on two consecutive days and diurnal variations in basal hormone fluctuations. Each patient showed unmeasurably low renin activity unresponsive to orthostasis and intravenous furosemide while plasma aldosterone was below normal range. Under ACTH-infusion only marked increases in aldosterone were observed in one patient whereas cortisol responded normally in all diabetics tested. Analysis of diurnal night day fluctuations (20.00-8.00) in plasma aldosterone and cortisol revealed a close and statistically significant relationship between both hormones in each of the three patients (p less then 0.05-less than 0.001). Variations in plasma aldosterone thus were mediated through changes in endogenous pituitary ACTH. Compared with normal controls however, diurnal aldosterone curves were set at a lower level. Our results demonstrate that a reduced sensitivity of the adrenal gland to ACTH is not responsible for the observed subnormal plasma aldosterone levels in these patients. Therefore, the lack of circulating angiotensin II seems to be the causative reason of hypoaldosteronism. The exact mechanism of undetectable renin activity in these patients remains unknown.