Seventeen neonates received an intravenous infusion of prostaglandin E1 for an average of 39 days (range 8 to 104). Seven (group 1) had transposition of the great arteries with no ventricular septal defect or a small one; eight (group 2) had ductus-dependent pulmonary flow (pulmonary atresia or stenosis in six and tricuspid atresia in two); and two (group 3) had aortic coarctation, one with no ventricular septal defect, the other with ventricular septal defect, isthmus hypoplasia and descending aortic flow supplied mainly by the ductus. An increase in the arterial partial pressure of oxygen (PO2) was seen in groups 1 and 2. Six patients from group 1 and two from group 2 developed heart failure; cortical hyperostosis of long bones was seen in three patients from group 1 and three from group 2; one from group 1 had refractory diarrhea. Other side effects seen at the beginning improved as the rate of infusion diminished. In group 3, the patient with complex coarctation had a decrease in blood pressure in the arms, an increase in pressure in the legs and restoration of renal function; in the patient with no ventricular septal defect, heart failure worsened during therapy. Histologic changes seen in three ductus were attributed to the closing process. When delaying surgery in selected ill infants with heart defects is deemed advantageous, long-term infusions of prostaglandin E1 are feasible.