Background Although plasma endothelin concentrations are elevated in patients with pulmonary hypertension, the precise sites of endothelin production have not been defined.We investigated the endothelin production in the pulmonary circulation of patients with mitral stenosis and its effects on pulmonary vascular tone.Methods and Results We measured plasma concentrations of endothelin-1, angiotensin II, and thrombomodulin in blood samples obtained from the right and left atria of 10 consecutive patients with rheumatic mitral stenosis (mean age, 55 years; range, 39 to 68) who were undergoing percutaneous mitral valvuloplasty. Plasma levels of endothelin-1 were significantly higher in the left atrium than in the right atrium (3.25+-0.45 versus 2.53+-0.36 pg/mL, mean+-SE, P<.001). The increased plasma endothelin-1 level in the left atrium, which reflected endothelin-1 production in the pulmonary circulation, was correlated with mean pulmonary artery pressure (r=.65, P=.04), mean pulmonary arterial wedge pressure (r=.67, P=.03), total pulmonary resistance (r=.68, P=.03), and 1/mitral valve area (r=.85, P=.002) but not with pulmonary vascular resistance (r=.04, P=.91). There were no significant differences in plasma levels of angiotensin II and thrombomodulin between the right and left atria (angiotensin II, 16.40+-3.08 versus 15.50+-4.85 pg/mL; thrombomodulin, 2.96+-0.34 versus 2.85+-0.37 ng/mL).Conclusions Endothelin-1 production is increased in the pulmonary circulation of patients with mitral stenosis in response to increased pulmonary artery pressure but is not directly related to increased pulmonary vascular tone in this disorder. (Circulation. 1994;89:2093-2098.)