OBJECTIVE--To study the dose response characteristics of prostaglandin E1 and epoprostenol (prostacyclin) and directly to compare their effectiveness as pulmonary vasodilators in infants with pulmonary hypertension. DESIGN--A crossover design with each patient receiving both drugs in random order. SETTING--Infants were studied in the intensive care unit while they were sedated, paralysed, and ventilated. PATIENTS--Twenty infants who had undergone corrective cardiac surgery and who were in sinus rhythm, had stable haemodynamic function, and had a pulmonary artery catheter in place. All infants were receiving dopamine and phenoxybenzamine. INTERVENTIONS--Baseline haemodynamic measurements were taken and an infusion of the first drug was started at the lowest dose: after 20 minutes the measurements were repeated and the dose increased. This protocol was repeated for all doses of both drugs: 10, 30, and 100 ng/kg/min of prostaglandin E1 and 5, 10, and 25 ng/kg/min of epoprostenol. Cardiac output was measured by the pulsed Doppler ultrasound method. MAIN OUTCOME MEASURES--Pulmonary and systemic vascular resistances were calculated from the cardiac output and compared by the Wilcoxon signed ranks test. RESULTS--Both prostaglandin E1 and epoprostenol were effective vasodilators: 5 ng/kg/min of epoprostenol was equivalent to 30 ng/kg/min of prostaglandin E1. CONCLUSIONS--Neither drug showed pulmonary specificity.