9 pre-term babies with patent ductus arteriosus (P.D.A.) and cardiac failure were found to have significantly higher plasma-levels of three prostaglandins (P.G.E, P.G.F, and P.G.FM) than a group of normal pre-term infants of the same age. There was no difference in the plasma-prostaglandin levels before and after surgery in the 5 infants who underwent ligation of the ductus, suggesting that the high P.G. levels are not the result of a patent ductus, but that P.G.S may have a role in the pathogenesis of P.D.A. 3 infants who were treated by indomethacin (a P.G. synthetase inhibitor) showed a sharp initial drop in plasma-levels of the three P.G.S, but levels of P.G.F and P.G.FM had risen either during or within 48 h of completion of the course of indomethacin. These results may explain why high failure-rates have been reported for treatment of P.D.A. by indomethacin.