The arrangement of extravertebral sources of spinal cord arteries in the rat is similar to that in man. Some differences, however, do exist, concerning mainly the arrangement of spinal branches supplied by the subclavian-vertebral arteries. Contrary to the situation in man, the vertebral artery in the rat appears to be the sole source of blood supply to the cervical cord. A high incidence of variations or anomalies of the vertebral artery in the rat existed. Vascular variations in the upper left thoracic spaces were frequent in the rat and more complicated than in man. In both species, a poor supply of significant spinal cord feeders from the superior intercostal artery of the costocervical trunk was found. Variations in the origin and branching of the aortic segmental arteries occur in both species. In man the major variation consists of two or more intercostal arteries arising from a common stem, while the rat more often displays a rudimentary subcostal artery. Finally, the ileolumbar arteries in the rat are constant branches of the lumbar aorta. Despite these differences in the vascular anatomy, the rat seems to be well suited for experimental comparative investigations, not feasible in man.