The hemodynamic and respiratory effects of phentolamine i.v. (15-30 mg/h) on pulmonary hypertension following chronic obstructive pulmonary disease (COPD) have been studied in 13 patients under stable conditions. Phentolamine produced a significant reduction in mean pulmonary arterial pressure, right and left ventricular filling pressures and right ventricular stroke work index. Cardiac index and oxygen delivery increased significantly. Individual analysis showed that in 7 patients the driving pressure decreased across the pulmonary circulation with concomitant increase in cardiac output, suggesting a direct vasodilating effect of phentolamine on the pulmonary circulation. Pulmonary gas exchange did not change significantly. In conclusion, in patients with pulmonary hypertension following COPD phentolamine given i.v. may have beneficial effects on right ventricular function and peripheral oxygen delivery without detrimental effect on gas exchange.