Disc gel electrophoretic patterns of esterases obtained in the buffer (0.1 M Tris-HCl buffer pH 7.4) extracts of the brains of twelve species belonging to various classes of vertebrates ranging from fishes to mammals are compared. Substrate specificity and inhibitor sensitivity is used to classify the enzymes into different classes of esterases. The pattern obtained does not suggest distribution of any specific enzymes to a particular class of vertebrates nor does it suggest any evolutionary change from fish to mammals. Some fast moving bands are observed in fishes and amphibians. Esterases with active thiol groups (Aryl and Esdp esterases) exhibit wide distribution among the species studied. The enzymes which are resistant to inhibition by eserine, pCMB and DFP (Acetyl and ER esterases) are present in the tetrapods studied.