1. Adrenomedullin is a potent vasodilating peptide first isolated from phaeochromocytoma and adrenal medulla but also found in the heart, lungs and kidneys. It may also be a paracrine factor because endothelial and smooth muscle cells synthesize adrenomedullin as well as express the receptors. Adrenomedullin induces vasorelaxation by activating adenylate cyclase and also by stimulating the release of nitric oxide. 2. We have developed a specific radioimmunoassay and measured the immunoreactivity of human adrenomedullin in the plasma of 58 male subjects: eight with essential hypertension, 12 with heart failure, 10 with ascites due to cirrhosis, 12 with chronic renal failure, four with hypoxia due to chronic obstructive pulmonary disease and 12 control subjects. 3. Plasma levels (mean +/- SEM) in patients with essential hypertension (16.3 +/- 1.9 pmol/l), congestive heart failure (17.5 +/- 2.8 pmol/l) and renal failure (17.7 +/- 2.5 pmol/l) were raised compared with control subjects (7.8 +/- 1.4 pmol/l, P < 0.05), confirming previous reports. 4. In addition, we observed that plasma levels of adrenomedullin were significantly raised in patients with ascites due to liver cirrhosis (15.5 +/- 1.9 pmol/l) and chronic obstructive pulmonary disease with hypoxia (20.0 +/- 1.5 pmol/l). 5. We concluded that the plasma level of adrenomedullin is raised in a variety of diseases.