The potential short-term pulmonary vasodilator effect of a calcium-channel blocker, nifedipine, was tested in seven patients with primary pulmonary hypertension. Nifedipine (20 mg) produced a significant (p<0.01) and persistent decrease in mean pulmonary arterial pressure (58.1 ± 14.3 to 48.6 ± 16.3 mm Hg) and pulmonary vascular resistance (1,070 ± 260 to 695 ± 266 dynes.sec.cm-5). Cardiac index increased from 2.5 ± 0.6 to 3.3 ± 0.8 L/min/m2 (p<0.01), and heart rate was unchanged despite a fall in systemic pressure. In three patients tested during exercise on a bicycle, nifedipine resulted in an increase in the duration of exercise in two and a blunting of the exercise-induced increase in pulmonary pressure in all three. Long-term treatment was initiated in five of the seven patients and in two additional patients who did not receive nifedipine in the short-term study; all but one experienced symptomatic improvement. A persistent hemodynamic improvement was observed in three of the four patients restudied after long-term therapy. In conclusion, this study demonstrated the short-term beneficial hemodynamic effects of nifedipine, both at rest and during exercise. A more extensive long-term follow-up is necessary to establish the usefulness of this drug in the treatment of primary pulmonary hypertension.