Interrupted aortic arch type B associated with ventricular septal defect (n = 17) or atrioventricular septal defect (n = 2) was induced in 19 of 300 fetuses by maternal administration of bisdiamine on the 9th and 10th day of pregnancy. After rapid whole-body freezing on the 21st day, the fetuses were studied by means of serial cross-sectional photographs of the frozen thorax. In fetuses with interrupted aortic arch, the ascending aorta was small, and the ratio of its diameter to the diameter of the pulmonary trunk was 0.64 ± 0.03 (mean ± SEM) in contrast to 0.96 ± 0.02 in the control. The aortic arch was interrupted distal to the left common carotid artery (type B of Cerolia and Patton) in all cas