Chronic enalapril therapy was assessed in 11 patients with cor pulmonale due to chronic obstructive pulmonary disease. Enalapril was added to the maintenance dose of diuretics and digitalis and when clinical stabilisation was achieved haemodynamics, spirometry, blood gases and maximal treadmill exercise test accompanied by +pulse oximetry were performed before and after 30 days, 10-20 mg a day, enalapril therapy. Haemodynamic study showed moderate but significant decrease in mean pulmonary artery pressure, from 24 +/- 3 to 21 +/- 5 mmHg (p = 0.05). There were no substantial differences in cardiac output as well as in blood gases and spirometry after enalapril therapy. Slight decrease in oxygen delivery, on an average from 9157 +/- 3808 to 8074 +/- 3574 (p = NS), was accompanied by a concomitant fall in haemoglobin. We noted significant improvement of maximal exercise test results after enalapril therapy. Maximal workload achieved and the time of exercise increased. It was accompanied by subjective improvement as assessed by Borg scale. We observed no adverse effects of enalapril during one month therapy in patients with cor pulmonale and COPD.