The formation and transformation of the pharyngeal arch arteries in the mouse embryo, from 8.5 to 13 days of gestation (DG), was observed using scanning electron microscopy of vascular casts and graphic reconstruction of 1-microm serial epoxy-resin sections. Late in 8.5-9DG (12 somites), the paired ventral aortae were connected to the dorsal aortae via a loop anterior to the foregut which we call the 'primitive aortic arch', as in the chick embryo. The primitive aortic arch extended cranio-caudally to be transformed into the primitive internal carotid artery, which in turn gave rise to the primitive maxillary artery and the arteries supplying the brain. The second pharyngeal arch artery (PAA) appeared late in 9-9.5DG (16-17 somites), and the ventral aorta bent dorsolaterally to form the first PAA anterior to the first pharyngeal pouch by early in 9.5-10DG (21-23 somites). The third PAA appeared early in 9.5-10DG (21-23 somites), the fourth late in 9.5-10DG (27-29 somites), and the sixth at 10DG (31-34 somites). By 10.5DG (35-39 somites), the first and second PAAs had been transformed into other arteries, and the third, fourth and sixth PAAs had developed well, though the PAA system still exhibited bilateral symmetry. By 13DG, the right sixth PAA had disappeared, and the remaining PAAs formed an aortic-arch system that was almost of the adult type.