The clinical syndrome of persistent pulmonary hypertension of the newborn includes a developmentally abnormal pulmonary microvasculature which contains excessive amounts of muscle and which cannot adapt to air breathing in the perinatal period. Surgical ligation of the ductus arteriosus of the fetal lamb has produced a physiologic model of pulmonary hypertension of the newborn. The aim of the present investigation is to determine whether surgical ligation of the ductus arteriosus in fetal sheep produces anatomic changes in the pulmonary blood vessels. The pulmonary vasculature of seven neonatal lambs that underwent surgical ligation of the ductus arteriosus from 6 to 17 d before birth was compared to that of five control lambs with a patent ductus arteriosus without fetal surgery and three control lambs with a patent ductus arteriosus that underwent sham surgery. Quantitative microscopic analysis of the barium gelatin-filled peripheral pulmonary vascular bed revealed an increase in the proportion of partially and fully muscularized pulmonary arteries at the level of the terminal bronchiole and within the acinus (p less than 0.0001). This finding demonstrates that medial muscle develops in areas of the distal pulmonary vascular bed where it is normally absent. Periadventitial fibrosis surrounding intraacinar pulmonary arteries was also present. No change in the number of small intraacinar arteries was detected. This structural remodeling of the peripheral pulmonary vascular bed was initiated in utero by ductus arteriosus occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)