A rat model was developed to examine changes in the posterior circle of Willis and the basilar and intracranial vertebral arteries after bilateral common carotid ligation. This procedure produced a major redistribution of blood to the head, with increased flow through the vertebral and basilar arteries. Changes in the vertebral, basilar, posterior communicating and proximal segments of the posterior cerebral arteries and neck vessels were assessed by postmortem barium sulfate arteriography and by histology of the middle portion of the basilar arteries serially at four days, and one, two, four and 15 weeks post-ligation. The changes noted were basilar and vertebral artery tortuosity, enlargement, and duplication of the vertebro-basilar junction. By 15 weeks, these intracranial vascular changes had largely regressed toward normal, commensurate with the appearance of multiple collateral vessels which were scattered throughout the soft tissues of the neck and shunted the original ligation sites. A mechanism that could explain these trophic vessel changes in response to increased blood flow is discussed. Some possible correlates of these findings with several brain vascular diseases are presented.