A patient with symptomatic primary pulmonary artery hypertension underwent therapeutic trials with parenteral and oral phentolamine. Before drug therapy, he manifested marked increases in pulmonary artery pressure and pulmonary arteriolar resistance during exercise. This hypertensive response to exercise was significantly attenuated by parenteral phentolamine. He also manifested sustained hemodynamic improvement with the same degree of attenuation of exercise-induced increases in pulmonary artery pressure and pulmonary arteriolar resistance 7 months after initiation of therapy with oral phentolamine. Twenty months after initiation of therapy, the patient continues to enjoy symptomatic improvement with a marked increase in exercise tolerance.