It has been demonstrated that a complex reorganization of the cellular architecture of the rabbit ductus arterious (DA) wall begins long before birth, breathing, and physiological contraction of the DA. This reorganization results in an intimal thickening which seems to make a significant contribution to the ultimate closure of the DNA lumen. The cellular events involved in the intimal thickening include reorientation and apparent intimal migration of the medial smooth muscle cells. These and other cellular aspects of DA closure are very similar to critical events occurring in the adult arterial wall during the early pathogenesis of intimal fibromuscular lesions (IFML) and closure of the DA serves as an excellent model of basic aspects of IFML formation. A continued light and electron microscopic study of the closure of the DA wall is proposed which aims to: 1) demonstrate through use of mitostatic drugs and quantitative techniques the role of mitosis in closure of the DA, 2) demonstrate through ultrastructural quantitation the possible role of decreasing innervation in the closure of the DA, and 3) investiate by a light and electron microscopic study the effects of prostaglandins, which are known to influence the patency of the DA, on the morphology of the DA wall. It is hoped that these studies will lead to a better understanding of factors controlling the cellular mechanisms involved in the beneficial closure of the DA. This new understanding may result in a more effective means of preventing the occurrence of both patent DA in human infants and IFML formation in adult arteries and vascular reconstructions.