During fetal life the pulmonary circulation is one of high resistance. Soon after birth with the onset of breathing and thereafter, sequential structural changes in pulmonary vasculature take place. Several authors have reported differently on this structural remodeling. This study describes the changes in the pulmonary vasculature occurring following birth. Twenty-five cases submitted to autopsy with ages ranging from newborns up to fourteen years were studied. Postmortem arteriography was undertaken in most of the cases and arterial morphology was quantitated by histomorphometry. It was found that the thick-walled pulmonary arteries of the newborn show a reduction in medial thickness over a period of one month, the reduction being most rapid in the first three days. After one month there is no significant change in medial thickness until fourteen years of age.