Histological and ultrastructural studies were made of 25 specimens of human ductus arteriosus obtained from abortion of autopsy, and ranging in age from 15 weeks of gestation to 7 years. Samples of ductus with normal muscular type structure exhibited active intimal thickening as early as 15 weeks' gestational age when the internal elastic lamina was found to be focally discontinuous. At the same time, intimal smooth muscle cells were closely arranged, and often intimately connected to cells of the tunica media. Ultrastructurally, both smooth muscle cells and endothelial cells at 15 weeks' gestational age contained abundant glycogen deposits. From 18-32 weeks' gestational age, glycogen deposits gradually disappeared, collagen fibres began to appear in the extracellular space and the first signs of smooth collagen fibres began to appear in the extracellular space and the first signs of smooth muscle cell degeneration became apparent. After birth, intimal thickening and degeneration of smooth muscle cells was much more pronounced. The ultrastructure of intimal smooth muscle cells indicated that intimal thickening was caused by smooth muscle cell migration as opposed to rapid proliferation; and both intracellular and extracellular membrane-bound lipid-filled vacuoles were commonly seen in the more advanced stages of degeneration. Of the ductus specimens examined, three were rich in elastic fibres. Two of these three specimens were from a group of 10 abortion cases, and the third was from a 2 weeks old full term infant who had been exposed to maternal rubella. The 2 weeks old infant had a widely patent ductus arteriosus; whether the two fetal specimens with ductus elastosis would have eventually developed into clinical patent ductus arteriosus was not clear.