On the basis of corrosion preparations and of microsphere studies, the following characteristics of the canine and feline cerebral circulation were observed. (1) In cats, a greater part of the vertebral arterial blood goes to the brain and it is more specifically restricted to the ponto-medullary and cerebellar structures. These structures received approximately 3 times more microspheres in cats than in dogs. (2) In dogs, an important amount of vertebral blood goes to the neck muscles, and the intracranial vertebral blood supply is spread over a greater area of the brain, including the thalamo-hypothalamic and posterior cortical zone. (3) In cats the thalamo-hypothalamic area receives a greater amount of blood via the common carotid artery than in dogs. (4) In both animal species, the vascular connections between the left and right side of the brain are more extensive in the vertebral than in the carotid bed. However, for either vascular bed, a more important left to right transmission was found in the dog.