Body fluid gas pressure and electrolytes of patients with ruptured aneurysm were continuously analyzed. Intracranial pressure (ICP) was regulated at the level of 120-100 mm H2O by cerebral ventricular drainage. There was no significant change in the pH, Pco2, Hco3, Na+, K+, Ca++ in the cerebrospinal fluid (CSF) of patients with slight or moderate disturbance of consciousness (lethargic-drowsy state). The Pcaro2 of the patients with marked disturbances of consciousness (semicoma-coma) was significantly low. PcarO2 of the patients with cerebral vasospasm was significantly lower than for those without vasospasms. PC8rO2/Pao2 was 0.27 ± 0.01 in the patients with vasospasm and 0.50 ± 0.01 in those with vasospasm. Pcaro2 tended to decrease in patients with markedly bloody CSF. When the bloody CSF was cleared by ventricular drainage, PCBfo2 increased. PcarO2 did not return to a normal value in the patients with marked disturbances of consciousness despite sufficient arterial oxygen tension. This suggests that Pcaro2 and Pcaro3/Pao2 should provide a convenient index for the prognosis of patients with ruptured aneurysm.