The pathogenesis of left or right aortic arch with aberrant subclavian artery was studied by making methacrylate casts and by immunohistochemistry using antidesmin, antimyosin and antifibronectin antibodies in the rat. Maternal rats were administered a total dose of 400 mg of N,N'-bis-(dichloroacetly)-diamine (bisdiamine) on day 9-10 after conception. It was demonstrated by statistical analysis that on day 18 after conception, the absorption rate was not significantly different between the controls and the group treated with bisdiamine. Left or right aortic arch with aberrant subclavian artery was seen in about 95% of the surviving embryos from the treated group. It was shown by the casts that the period of obliteration of the dorsal aorta in the embryos from the treated group was half day earlier than that of the control embryos. In the control embryos, the left 7th intersegmental artery moved to the proximal dorsal aorta to form the left subclavian artery during development. In the embryos from the treated group, such movement of the 7th intersegmental artery was retarded. Antimyosin staining demonstrated certain cells, presumably neural crest cells, migrating to the tunica media of the dorsal aorta. After these cells reached the developing tunica media, the immunoreactivities to the antimyosin and antidesmin antibodies appeared in the tunica media. The period of appearance of the immunoreactivities in the left dorsal aorta corresponded to the period of obliteration of the right dorsal aorta in the control embryos. In the embryos from the treated group, the appearance of immunoreactivities to the antidesmin and antimyosin antibodies was half to one day later than that of the control embryos. Antifibronectin staining demonstrated that fibronectin accumulated in the developing tunica media and disappeared from the tunica media. In the embryos from the treated group, however, the disappearance of fibronectin was one day earlier than that of the control embryos. From these results, it is suggested that the obliteration of the dorsal aorta is regulated by certain cells, probably neural crest cells and that the beginning of differentiation and function of the tunica media is regulated by these cells. Bisdiamine might inhibit the action or migration of the cells.