The morphogenesis of the aortic arch system was studied in rat embryos whose mothers had been treated with glycerol formal (GF) during pregnancy. The fetuses were removed from the uterus on days 13-17 of gestation and India ink was injected into their vascular system while they were still alive. The teratogenic effect of GF manifested itself by developmental retardation of the aortic arch system, vessel dilatation and persistence or anomalous regression of embryonic vessels. These conditions result in several aortic arch malformations: right aortic arch, double aortic arch and persistence of the 3rd aortic arch.