The effects of Captopril, an angiotensin-converting enzyme inhibitor, on pulmonary hemodynamics and blood gases were studied in 9 patients with chronic obstructive lung disease (COLD) and pulmonary hypertension (PA-P greater than 20 mm Hg). Hemodynamic data were recorded prior to Captopril administration (50 mg per os) and for the next 60 min. Following Captopril administration, significant reductions in mean pulmonary artery pressure (PA-P) (p less than 0.05), in mean pulmonary wedge pressure (PW-P) (p less than 0.05), and in total pulmonary resistance (TPR) were noted; significant reductions in mean brachial artery pressure (BA-P) and systemic vascular resistance (SVR) were also recorded, while cardiac output, heart rate and blood gas tensions showed no significant changes. Furthermore, the higher the hypoxemia, the greater was the reduction in BA-P (p less than 0.05). We therefore feel that Captopril, when administered to COLD patients with pulmonary hypertension, may protect the pulmonary circulation from hypoxic pulmonary vasoconstriction.