At least three forms of phenyl valerate esterases are present in hen brain cytoplasmic microtubules (MT). Thermostability studies reveal two additional forms in brain homogenates of cow, mouse, pig, rabbit and rat. The distribution of these brain esterases is not related to the age of the hens or the susceptibility of the species to organophosphate (OP)-induced delayed neurotoxicity. MT phenyl valerate esterases are distinct enzymes from the MT-associated proteases degrading high-molecular weight MT-associated proteins (hmw MAPs). Hen brain and spinal cord lysosomes on in vitro incubation release phenyl valerate esterase(s) and hmw MAPs-protease(s). OP neurotoxicants act in vitro to stabilize rat but not hen brain lysosomes. In vivo studies with hen brain and spinal cord lysosomes indicate that OP-induced delayed neurotoxicity is not initiated by disruption of lysosomal stability.