Adrenomedullin (AM) is a potent vasodilative and natriuretic peptide that is processed from its precursor as the intermediate form, AM-glycine-COOH (iAM). Subsequently, iAM is converted to the biologically active mature form, AM(1-52)-CONH<sub>2</sub> (mAM), by enzymatic amidation. Using immunoradiometric assays that recognize total AM (tAM) and only mAM, we determined the plasma and urinary levels of mAM and iAM in patients with chronic glomerulonephritis (CGN). The plasma mAM concentration was significantly higher in the patients than in the controls (1.8 ± 0.1 vs. 1.3 ± 0.1 fmol/ml, p < 0.01), whereas the plasma iAM concentration of the CGN patients did not significantly differ from that of the controls (9.4 ± 0.5 vs. 8.9 ± 0.5 fmol/ml). Levels of urinary mAM excretion in the patients did not statistically differ from those of the controls (1.6 ± 0.4 vs. 2.0 ± 0.3 fmol/mg creatinine), whereas urinary iAM excretion was significantly lower in the CGN patients (3.7 ± 0.7 vs. 5.6 ± 0.8 fmol/mg creatinine, p < 0.05). Urinary excretion levels of mAM significantly correlated with those of sodium (r = 0.47, p < 0.05), whereas those of iAM did not. In conclusion, the plasma ratio of mAM to iAM is augmented in CGN patients, and mAM appears to be involved in the regulation of sodium. Therefore, determination of the mAM in addition to the tAM concentration is essential in CGN patients.