The Penfluridol (PFL) is a potent and long-acting neuroleptic belonging to the diphenylbutyl piperidine series. The purpose of the present study is to investigate a correlation between the brain concentration of unchanged 3H-PFL and its neuroleptic activity in the methamphetamine-antagonism test. The experimental animal comprised adult male Wistar rats weighing about 200 g. All the rats were administered 3H-PFL orally and killed at appropriate times after administration. After killing blood samples were collected and the brain and liver were excised. The blood was extracted with 4 volumes of ethanol. The brain and the liver were homogenized in 9 volumes of ethanol. The amount of 3H-PFL in the ethanol extract was determined by thin-layer chromatography. The result showed that the drug level in the brain was relatively constant when the dose was the median effective dose (ED50) for the test at those time intervals after administration.