Chronic pulmonary arterial hypertension was produced in six fetal lambs. In four (126 to 139 days' gestation) unilateral fetal renal artery constriction caused systemic arterial mean blood pressure elevations. In another fetus, constriction of the umbilical artery caused a systemic mean blood pressure elevation; in the sixth, partial occlusion of the ductus arteriosus caused isolated pulmonary arterial hypertension. The right lung of each fetus was perfused with fixative at the in vivo mean arterial pressure and the amount of smooth muscle in the fifth generation (resistance) vessels analyzed using the medial width/external diameter ratio. There was a significant increase in the medial width/external diameter ratio in the six experimental animals as compared to that in six normal fetuses. In separate fetuses the increased ratios were due to a decreased external diameter, increased smooth muscle, or both these factors. The total number of resistance vessels was counted in the right lung of each fetus and no significant difference from normal was observed. We postulate that either fetal systemic hypertension or constriction of the ductus arteriosus causes fetal pulmonary hypertension in utero and that this produces increased smooth muscle development in pulmonary arterial resistance vessels; this may be a pathogenic mechanism for the syndrome of persistent pulmonary hypertension of the newborn infant.