Vasodilators including isoproterenol, nitroprusside, nitroglycerin, and oxygen were administered to 5 patients with chronic pulmonary hypertension secondary to pulmonary embolism. The patients' mean pulmonary artery pressure was 54 +/- 8 mmHg, and their average total pulmonary resistance (TRP) was 17 +/- 11 mmHg/L/min. Each patient experienced a decrease in TRP in response to at least 1 of the vasodilators, and the mean maximal decrease was to 57 +/- 10% of the baseline value. Although these decrements in TRP were accompanied by an increase in cardiac output in all but 1 patient, changes in PAP were variable. Treatment with nitroglycerin was continued in 3 of the 5 patients and all 3 have reported improvement in their exercise tolerance. We conclude that the elevated TPR seen in patients with chronic embolic pulmonary hypertension is due to increased vascular tone in addition to fixed vascular obstruction.