We have obtained dose-response curves for the effects of prostacyclin on the pulmonary and systemic circulations of 10 patients with pulmonary hypertension secondary to congenital heart disease. With the subjects breathing air, prostacyclin caused statistically significant, pulmonary and systemic vasodilation. When the patients breathed 100% oxygen, pulmonary blood flow rose and pulmonary vascular resistance fell with no significant change in pulmonary artery pressure. Prostacyclin had a small additional vasodilator effect, but this did not reach statistical significance. Prostacyclin may be of some use in the assessment of the reversibility of an elevated pulmonary vascular resistance when surgery for the underlying congenital heart defect is being contemplated.