Adrenomedullin (AM), a potent vasodilator and natriuretic peptide, is found in human blood. To investigate the pathophysiological role of AM in essential and malignant hypertension (EHT and MHT), we measured the plasma concentrations of AM in patients with EHT of WHO stage I or II (n=42) and in those with MHT (n=9) by a specific radioimmunoassay, and compared these concentrations with those in normotensive controls (n=46). The plasma concentrations of atrial and brain natriuretic peptides (ANP and BNP) in these subjects were also measured by immunoradiometric assays, and their relations to plasma AM were examined. The plasma AM level in the EHT patients (7.15±0.21pmol/l, mean±SEM) was significantly (p<0.01) higher than that in the normotensive controls (6.14±0.25 pmol/l), and a further elevation was observed in the MHT patients (14.1±3.8pmol/l). Similar elevations of plasma ANP and BNP were seen in the two patient groups. The plasma AM level significantly (p <0.01) correlated with not only the systolic (r=0.44) and diastolic (r=0.46) blood pressures, but also with the plasma levels of ANP (r=0.43) and BNP (r=0.43). The elevated plasma concentration of AM in the MHT patients decreased significantly (p<0.05) after antihypertensive treatment, and the plasma ANP and BNP levels similarly declined. These results suggest that AM may participate, along with ANP and BNP, in mechanisms counteracting a further elevation of blood pressure in patients with EHT and MHT. (Hypertens Res 1999; 22: 61-65)