The effect of prostaglandin E1 (PGE1) on the haemodynamics during cardiac catheterisation was studied in 6 newborn infants with pulmonary atresia and ductus-dependent pulmonary perfusion. In 4 patients with initial arterial O2-saturations between 24% and 45% pulmonary blood flow increased from a mean value of 1.0 to 4.5 1/min/m2, 15--20 minutes after the infusion of PGE1 was started. At the same time the systemic blood flow decreased markedly, the systemic blood pressure varied only slightly, and a marked increase in systemic vascular resistance was calculated. In one further patient with a relatively high initial arterial O2 saturation the pulmonary blood flow increased only minimally, whereas in the remainder patient with low initial arterial oxygen saturation there was no change after PGE1 during the investigation.