Fourteen patients, 2 to 20 years old were investigated. Two had primary pulmonary hypertension, 11 had congenital heart disease and post-tricuspid shunts, and 1, a 20-year-old patient, was investigated after he had undergone surgical correction of truncus arteriosus I. Pulmonary arterial pressure, pulmonary flow index, peripheral systolic blood pressure and heart rate were measured before, and several times after intrapulmonary injection into the pulmonary artery of 0.5 pgnifedipine/kg. Six patients were given an additional dose of ! pg nifedipine per kilogram into the pulmonary artery and hemodynamic measurements were repeated. In eight children, receiving 100% oxygen via a breathing mask, nifedipine effects were compared with oxygen effects. After 10 minutes under oxygen, the same bemodynamics were determined as after nifedipine. In addition, in four of these children aortic pressure and arterial oxygen saturation were also measured. Maximal effects occurred within 4 minutes. 0.5 ~g nifedipine per kilogram caused a slight reduction in mean pulmonary arterial pressure (p < 0.05), as well as increase in pulmonary flow index (p < 0.005). However, no significant change in heart rate or in systolic blood pressure was observed. 1 pg nifedipine per kilogram IP had almost the same effects. No adverse side effects occurred, besides mild headaches in one child. A comparison of nifedipine injected into the pulmonary artery with oxygen breathing in congenital heart disease combined with pulmonary hypertension, is reported for the first time. Nifedipine had a more pronounced and beneficial effect with a selective action on the pulmonary vascular bed.