Assessment of mineralocorticoid replacement therapy in Addison's disease relies on clinical features and laboratory measurements, including plasma renin and potassium. Previous studies have questioned the value of measuring the plasma renin concentration (PRC), particularly in the setting of fludrocortisone overreplacement. The aim of this study was to evaluate the usefulness of plasma atrial natriuretic peptide (ANP) measurements as a marker of sodium and volume status in Addison's disease. Fourteen patients with Addison's disease receiving their usual glucocorticoid doses were placed on various doses of fludrocortisone (FC; 0 mg, 0.05 mg, 0.1 mg and 0.2 mg) in random order for four 2-week periods. At the end of each period, blood pressure and clinical symptoms were assessed, and blood was drawn for measurement of PRC and ANP levels. PRC was significantly elevated in patients receiving placebo (54.2 +/- 57.9 ng/mL x h) compared with PRC in those receiving baseline FC (24.7 +/- 42.4 ng/mL x h), 0.1 mg FC (15.2 +/- 25.9 ng/mL x h), and 0.2 mg FC (5.5 +/- 5.7 ng/mL x h). ANP levels were measured by either an extraction method (ANP(ext)) or directly from plasma (ANP(dir)). ANP(dir) was significantly elevated at 0.2 mg FC (87.1 +/- 20.1 pg/mL) compared with baseline (63.3 +/- 8.1 pg/mL), placebo (56.1 +/- 5.5 pg/mL), 0.05 mg FC (60.5 +/- 16.0 pg/mL), and 0.1 mg FC (65.4 +/- 13.7 pg/mL) values. ANP(ext) was elevated in patients receiving 0.2 mg FC (42.7 +/- 41.8 pg/mL) compared with that in patients receiving placebo (7.9 +/- 5.4 pg/mL), 0.05 mg FC (16.2 +/- 11.2 pg/mL), or 0.1 mg FC (19.7 +/- 11.1 pg/mL). Our data suggest that PRC is of value in determining mineralocorticoid underreplacement, whereas ANP is a more sensitive index of FC overreplacement. ANP levels may, therefore, be complementary to PRC in adjustment of mineralocorticoid doses in the upper dose range, where clinical symptoms and signs appear to be of little value.