Hemodynamic parameters were measured by ballon tipped catheter in 11 patients with COPD and plasma levels of endothelin1 (ET1), cAMP and cGMP were detected by radioimmunoassay. The results indicated that 3,4-dihydroxyacetophenone (DHAP) gives with an i.v. dose of 640mg could reduce (1) 15% of mean pulmonary arterial pressure, 39% of pulmonary vascular resistance and 21% of systemic vascular resistance; (2) but not effect on systemic arterial pressure and blood gas analysis; (3) decrease the plasma ET1 as well as cGMP level (P < 0.01, P < 0.05 respectively). The authors think that the decrease of pulmonary arterial pressure in COPD patients may be related to the lowering of plasma ET1 level and the change of the cAMP to cGMP ratio.