In patients undergoing intracranial surgery, the drugs used for anesthesia and the opera-tive procedure can effect an increase in intracranial pressure. Especially in patients whose intracranial pressure has already increased significantly, if the pressure is further increased br these factors, then the patient's neurological condition may be aggravated and the post-operative prognosis may be affected. Thus it is desirable to use drugs which reduce intrac-ranial pressure during this type of surgery. In this study, the spinal subarachnoidal pressures of two groups were compared. One group received a combination of thiopental sodium and lidocaine which are known to relatively reduce intracranial pressure, white the other group was given halothane which is known to increase intracranial pressure. The summary of this study is as follows : 1) The spinal subarachnoidal pressure began to increase 1~4 minutes after the administra-tion of halothane, and the duration of increament was 15-38 minutes with a maximal pressure change from 346+/-63 mmHaO to 417+/-90 mmH2O. Thereafter the pressure returned to the pre-induction level spoataneously. 2) The spinal subarachnoidal pressure began to decrease 2~4 minutes after the admini-stration of thiopental sodium and lidocaine. The greatest decrease in pressure occurred within 10~15 minutes level as the Preseure fell form 324+/-41 mmH2O to 210+/-73 mmH2O, thereafter the pressure remained decreased.