AbstractThe level of neurotoxic esterase in brain, spinal cord and platelets of certain birds and mammals has been determined. The enzyme activity for the birds was maximal in hens and for the mammals was maximal in rats. The activity decreased progressively in the cerebral cortex, corpus striatum, spinal cord and blood platelets of the birds as well as the mammals. The difference in the susceptibility of birds and mammals to organophosphate‐induced delayed neurotoxicity may be related to differences in the relative concentrations of the enzyme in the different specie