We studied 17 fetal rats with vascular ring composed of the left aortic arch and right ductus arteriosus and 7 fetal rats with the mirror-image arrangement. The vascular ring of this type was induced by administration of bis-diamine (200 mg) on the 9th and 10th day of pregnancy in rats. In the most severe case, the esophagus and trachea were compressed completely and no inner space was left, and localized and diffuse tracheal stenosis was prominent. The fetal lung mass was increased as the stenosis of the ring was severe. The thoracic descending aorta was positioned in the left side, and was contrasted with the clinically observed right descending aorta in this postnatal anomaly. Thus we are reporting two new findings in the fetal vascular ring with the left aortic arch and the right ductus arteriosus: increased lung mass and left descending aorta in a fetal animal model.