Abstract. We attempted to answer to the question whether excessive rises in endogenous plasma angiotensin II (All) stimulate ACTH secretion by measuring PRA, All, AVP, ACTH, and cortisol in 8 patients with Addison's disease before and after withdrawal of fludrocortisone substitution. Blood was drawn at 14.30 h, exactly 6½ h after the morning dose of hydrocortisone had been taken. PRA and All were initially higher than normal in 4 patients. After withdrawal of fludrocortisone for 1 or 2 weeks, PRA and All rose markedly in 4 patients (up to 260 ng/l) without concomitant changes in plasma ACTH levels (r = −0.081, All vs ACTH). Changes in plasma cortisol could not have obscured a stimulatory effect of All on ACTH by variable feedback inhibition of ACTH release. The increase in plasma All levels in the 4 patients was larger than that observed in a previous study in normal subjects after rigorous dietary sodium restriction. In all patients, hyperkalaemia developed after fludrocortisone withdrawal, independent of changes in PRA and AII. Rises in PRA, All, and plasma potassium were partially reversed by increased sodium intake and further suppressed by resumption of fludrocortisone therapy. Plasma AVP remained in the normal range after fludrocortisone withdrawal, but was slightly elevated after increasing salt intake without fludrocortisone administration. Conclusions: 1) Rises of endogenous plasma All to levels tenfold higher than normal do not stimulate ACTH release. All is probably not a physiological modulator of ACTH secretion. 2) Mineralocorticoid substitution in Addison's disease should be monitored by plasma potassium measurement. Hyperkalaemia may coexist with normal PRA.