Objective: This article specifically addresses the question of whether the manipulable cervical lesion is likely to cause extrinsic compression of the vertebral arteries sufficient to cause such symptoms of reduced regional cerebral blood flow as might be relieved by spinal manipulation. Data sources: Literature on normal and abnormal cerebral circulation, including vertebrobasilar insufficiency. Data synthesis: Signs and symptoms produced by extrinsic compression of the vertebrobasilar system have been compared with those attributed elsewhere to the manipulable cervical lesion (cervical subluxation). Results: Extrinsic compression of the vertebrobasilar system generally does not produce signs and symptoms consistent with those attributed to the manipulable cervical lesion. Conclusion: It has been hypothesized elsewhere that the manipulable cervical lesion may induce localized decreases in regional cerebral blood flow, and so signs and symptoms attributable to "cerebral hibernation." If a causal relationship does exist between the cervical subluxation and reduced regional cerebral blood flow, it is not likely to be caused by mechanical compression of the vertebral arteries.