Isoflurane is often prefered to for neuroanesthesia because it has the least effect on cerebral blood flow (CBF) and cerebrospinal fluid pressure changes. The author performed this study to compare the change of the intracranial pressure following the increasing concentration of isoflurane in 10 cats. The results were as follows, 1) 1ntracranial pressure was increased significantly with 1.5% (p<0.05) and 2.0% (p < 0.005) isoflurane administration, but there was no significant increase with administration of 1.0% isoflurane. 2) Cerebral perfusion pressure was decreased significantly with 1.0% (p<0.01), 1.5% (p<0.005) and 2.0% (p<0.005) isoflurane administration. So it is recommend to administrate less than 1.0% isoflurane in increased intracranial pressure patient.