The effects of homopantothenic acid (HOPA) on the brain were invetigated electrophysio-logically. Local application and intra-arterial administration of the drug did not affect the trans-callosal potentials of the cats anesthetized with sodium pentobarbital. This suggests that the action of HOPA in the central nervous system does not depend upon γ-aminobutyric acid released from HOPA itself. The spontaneous EEG of the unanesthetized rabbits restrained stereotaxically was slightly changed into the alert pattern by HOPA. The intra-venous administ-ration of 500 mg/kg of HOPA did not antagonized pentylenetetrazol or picrotoxin induced seizures, but when 5 mg/kg of HOPA was administered intra-cisternally, these seizure discharges were arrested. The fact shows that HOPA hardly passes through the blood brain barrier.