1. The accumulation of p-chlorophenylethylamine (pCPE) in rat brain after administration of pargyline plus p-chlorophenylalanine (pCPA) is demonstrated. 2. Measurements of pCPE in brain were performed at 0.25 h, 1 h and 4 h after administration of pCPA to pargyline-pretreated rats, and amounts of pCPE were shown to increase with time.