The effects of oxygen inhalation at atmospheric pressure (1 ATA.O2) on CBF were studied in subarachnoid hemorrhage (SAH) patients due to ruptured intracranial aneurysms to prove the usefulness of the "O2 response" test for the evaluation of the cerebral vascular response. "O2 response" means the CBF (or ICP) decrease during hyperoxemia. CBF was measured by 10 m Ci 133Xe intravenous injection method using rCBF analyzer BI-1400 (Valmet). Two-compartmental analysis was used for the calculation of initial slope index (ISI). Studied cases were 53 postoperative SAH patients and 100 times examinations were done under the condition of Rest-1 ATA.O2. The incidence of global disturbance of the O2 response was 33% from day 0 to 3, 31% during day 4 to 7, 30% during day 8 to 14, and 23% at day 15 to 30. But, after day 31, there was no case which revealed an impaired O2 response. It was not possible to detect focal or hemispheric abnormalities of the O2 response because of the limitation of noninvasive two-dimensional CBF measurement method employed. The causes of O2 response abnormality, within a month after the onset, were increased ICP, hydrocephalus and diffuse brain damage. Multiple regression analysis proved that elevated PaO2 is the major factor in reducing CBF. CBF and ICP correlation study showed that the higher the Rest ICP, the fewer the CBF decreases during pure O2 inhalation. This may indicate that the increase in ICP in the acute stage changes the O2 response according to the level of increased ICP.(ABSTRACT TRUNCATED AT 250 WORDS)