Three patients who had large, benign cerebellar tumors were operated upon in the sitting position and developed symptoms referable to the temporoparieto-occipital region of the brain 24-48 h postoperatively. They consisted of dizziness, nausea, vomiting, formed and unformed hallucinations, and inversion of image or disorientation in space, some of which were experienced by some of the astronauts and cosmonauts during space flight. Such findings are not due to stimulation of the cerebellum, the site of the lesion, but must come from the cerebral hemisphere. The symptoms were believed to be caused by "the luxury perfusion" of Lassen with the development of local lactic acidosis secondary to vascular insufficiency to the brain in the distribution of the posterior cerebral artery thus stimulating the temporoparieto-occipital region. This theory is suggested to some degree by the work of Endo et al. using CT scans, which showed the shifting of increased blood flow from the frontal region to the temporoparieto-occipital region following removal of a benign posterior fossa tumor. The mechanism for the compression of the posterior cerebral artery may be due to uncal herniation at the tentorium. The authors believe that it might be well to consider further testing in a vertical or oblique plane rather than only in a centrifugal horizontal one. This method would tend to cause uncal herniation more readily. Monitoring of such effects could be done with the colored CT scan.