Captopril, a potent inhibitor of angiotensin converting enzyme, was tested in patients with COPD (means forced expired volume in the first second--FEV1 = 0.73 l) and pulmonary hypertension (PAP = 41.3 mmHg). In the first phase of the experiment, patients underwent and incremental exercise test to the limit of tolerance. These were double blind, randomized, cross-over studies, where the patients received oral placebo (Pl) or captopril (Cp) 25 mg, on different days. In a second phase, the patients were submitted to hemodynamic and gasometric studies in the supine position, before placebo, the 60 min after and immediately after exercise (cycling-like leg movements). After 30 min of rest the same protocol was repeated with oral administration of 25 mg of captopril. In the metabolic evaluation (cycloergometry) captopril increased significantly exercise tolerance (means VO2-uptake at maximal exercise: CP = 0.81 vs Pl = 0.73 1/min), associated with a slower heart rate and higher O2-pulse at maximal exercise. In the hemodynamic study, when the effects of Cp and Pl were compared, the mean values of pulmonary artery pressure (PAP) and pulmonary vascular resistance (PVR) were similar at rest, but significantly lower during exercise, after captopril (means PAP Cp = 41.3 vs Pl = 51.2 mmHg; XPVR Cp = 278 vs Pl = 392 dyn. sec. cm5). There were similar systemic hemodynamic effects after Cp, but these were more intense in the pulmonary circulation (lower PVR/SVR ratio post-Cp in relation to post-Pl, during exercise). The cardiac index, systemic O2 transport and arterial and mixed venous blood gases were similar at rest and during exercise, with Pl or Cp.(ABSTRACT TRUNCATED AT 250 WORDS)