In order to elucidate some of the unexplained phenomena in prolonged patency of the ductus arteriosus in preterm infants, the histology of the ductus was studied in 27 cases. Some of the infants had been treated with indomethacin. Four morphologic maturation stages are distinguished. There was no strict relation between gestational age or birth weight and histologic maturation. Therefore, one cannot predict whether a ductus is likely to be mature at the time of birth. In all infants with clinically diagnosed prolonged patency of the ductus beyond the first week, the immature maturation stage or the permanent patent type was observed. In both stages, reopening after initial closure with indomethacin occurred.