AbstractThe influence of bis‐diamine on truncoconal septation was studied by the administration of 200 mg of this drug to pregnant Donryu‐strain rats on day 10 of gestation. Fetuses were removed at nine time periods from day 11 to 15. Some hearts were observed under a scanning electron microscope; others were serially sectioned and observed by light microscopy. In the treated group, the aorticopulmonary septum and the 6th aortic arch artery were hypoplastic or aplastic. Truncal swellings and conal ridges showed hypoplasia and failure to fuse. Various types of persistent truncus arteriosus were found in association with the degree of development of the 6th aortic arch artery and the aorticopulmonary septum. Fused 6th aortic arch arteries corresponded to Collett and Edwards' persistent truncus arteriosus type II, and non‐fused arteries to type III. Aplastic arteries belonged to type IV or were unclassifiable.