Captopril 12.5 to 50 mg as a single dose was given to six patients with pulmonary hypertension secondary to collagen vascular disease. Total pulmonary resistance was decreased in four patients from 19% to 39%, but mean pulmonary artery pressure (63 +/- 15 mm Hg) was not decreased, probably because of the concurrent increase in cardiac output from 21% to 52% (2p less than 0.05). The systemic arterial pressure was slightly decreased due to the decrease in total systemic resistance. Control plasma renin activity ranged from 0.15 to 16 ng/ml/hr and was increased during captopril from 19% to 356%. These results, although preliminary, suggest that captopril may be beneficial in certain patients with pulmonary hypertension secondary to collagen vascular disease.