Adrenomedullin (AM), a novel hypotensive peptide, preferentially dilates pulmonary vessels rather than systemic vessels. This suggests the possibility that AM is a circulating hormone which participates in regulation of the pulmonary circulation. A recent study revealed that two molecular forms of AM, i.e. a mature, active form of AM (AM-m) and an intermediate, inactive, glycine-extended form of AM (AM-Gly), circulate in human plasma. In the present study we investigated the production and clearance sites and pathophysiological significance of the two molecular forms of AM in the pulmonary circulation in patients with mitral stenosis. We measured the plasma levels of AM-m and total AM (AM-T; AM-m+AM-Gly) using a recently developed specific immunoradiometric assay, and thus calculated plasma AM-Gly levels, in blood samples obtained from the femoral vein, pulmonary artery, left atrium and aorta of 28 consecutive patients with mitral stenosis (20 females and eight males; age 53+/-10 years). Patients with mitral stenosis had significantly higher venous concentrations of AM-T, AM-Gly and AM-m than age-matched normal controls (AM-T, 15.9+/-2.5 and 10.6+/-2.1 pmol/l respectively; AM-Gly, 14.0+/-2.1 and 9.8+/-1.9 pmol/l respectively; AM-m, 1.9+/-0.6 and 1.1+/-0.3 pmol/l respectively; each P<0.001). There was a significant decrease in the concentrations of AM-m and AM-T between the pulmonary artery and the left atrium (AM-T, 16.1+/-2.7 and 14.0+/-2.4 pmol/l respectively; AM-m, 2.0+/-0.6 and 0.7+/-0.2 pmol/l respectively; each P<0.001); however, there were no differences in plasma AM-Gly levels between the pulmonary artery and the left atrium (14.1+/-2.3 and 13.5+/-2.3 pmol/l respectively). The venous concentrations of AM-m, AM-Gly and AM-T showed similar correlations with mean pulmonary artery pressure (AM-T, r=0.67; AM-Gly, r=0.63; AM-m, r=0.59; each P<0.001) and total pulmonary vascular resistance (AM-T, r=0.77; AM-Gly, r=0.70; AM-m, r=0.75; each P<0.001). These results suggest that the plasma concentration of AM-m is increased in parallel with those of AM-Gly and AM-T, and that the main site for clearance of AM-m from the plasma is the lung; the extracted AM-m in the lungs may help to attenuate the increased pulmonary arterial resistance in secondary pulmonary hypertension due to mitral stenosis.