Diodrast was injected into the right vertebral artery of a living subject and films exposed during the phase of arterial filling. The vertebral artery is visible in relation to the axis and atlas (12), and as it passes through the foramen magnum. The posterior inferior cerebellar branch (11) is given off before the vertebral arteries unite to form the basilar artery (4). The basilar artery terminates by branching into the paired superior cerebellar arteries (2, 3) and the paired posterior cerebral arteries (7, 9). Branches of the latter which pass to the inferior surfaces of the temporal and occipital lobes are indistinct. (These films have been reproduced through the courtesy of Dr. Earl Miller of the University of California Hospital.)