Two types of cholinesterase (ChE) have been found in brain and whole blood. These are (1) true cholinesterase (TChE), also known as acetylcholinesterase, which acts preferentially on acetylcholine (ACh) and is found chiefly in the gray matter (neurons) of the brain and in red blood cells, and (2) pseudocholinesterase (PChE), also known as butyrylcholinesterase, which acts preferentially on butyrylcholine (BuCh) and is found chiefly in the white matter (glial cells) of the brain and in blood serum. TChE is believed to be related to conduction in brain and nerve; in red blood cells it may be concerned with regulation of cell permeability. The functions of PChE in brain, nerve, and blood serum are all still obscure.1,2 Recently, Desmedt and La Grutta3 demonstrated that pharmacological “arousal” reactions in the brain result from selective inhibition of PChE preferentially to TChE. This recalls to mind the observation4