The ductus arteriosus in mice was studied by light and electron microscopy for a certain time span (from day 15 of gestation to 16 weeks after birth). In the intima, subendothelial intimal cells began to appear at day 17 of gestation, while actual constriction of the ductus arteriosus progressed rapidly after birth, beginning always at the site adjacent to the ductus arteriosus-aorta junction. At 3 h postnatally, the ductus arteriosus showed complete occlusion (functional closure), which was due mainly to constriction of the medial smooth muscle cells. At this stage, intimal cells (endothelial and subendothelial intimal cells), which occupied the ductal lumen, demonstrated ultrastructural features of undifferentiated cells. However, all the intimal cells 3 weeks after birth and thereafter were revealed to have electron-microscopic features of mature smooth muscle cells. The smooth muscle cells of the media and intima decreased progressively in number with advancing deposition of stromal collagen and elastic fibers in the ductal wall.