Changes in distribution of elastin and elastin receptor were studied during maturation of the ductus arteriosus. In this vessel intimal thickening is a normal process. It starts with separation of endothelial cells from the internal elastic lamina by the formation of a wide subendothelial region, followed by an increase in number of radially orientated cells in the inner media and subendothelial region. For the first time the nature of these originally inner media cells could be definitely determined as being derived from smooth muscle cells. Areas rich in these modified smooth muscle cells contained less elastin as compared with regions rich in circularly orientated smooth muscle cells. The internal elastic lamina had disappeared underneath intimal cushions of the canine ductus arteriosus, whereas in the fetal human ductus arteriosus, splitting of the internal elastic lamina was observed. Elastin was not present underneath separated endothelial cells, which suggests that these cells do not synthesize elastin, at least not after separation. Both smooth muscle cells and endothelial cells demonstrated the presence of the elastin receptor. Changes in its distribution were not observed. The temporal and spatial association between the altering distribution of elastin and the absence of normal cushion formation in the persistent ductus arteriosus suggests a role of the elastin receptor in the process of intimal thickening.