Organophosphorus and carbamate pesticides are cholinesterase (ChE) inhibiting chemicals that have been responsible for avian die-offs. Identification of chemicals implicated in these die-offs is difficult and sometimes conclusions are solely circumstantial. However, when marked depression (inhibition) of brain ChE activity accompanies organophosphorus or carbamate residues in body tissues or ingesta, cause-effect diagnosis is enhanced. To achieve this end, normal brain ChE activity is estimated for controls of the affected species and then die-off specimens are individually evaluated for evidence of ChE inhibition. This approach to evaluation of antiChE poisoning may also be used to monitor exposure of vertebrates to field application of organophosphorus or carbamate pesticides. Problems associated with this kind of evaluation, and the main topic of this report, include variability of brain ChE activity among species, postmortem influences of ambient conditions (storage or field) on ChE activity, and differential patterns of ChE activity when inhibited by organophosphorus or carbamate compounds. Other topics discussed are the ChE assay procedure, example case reports and interpretation, and research needed for improving the diagnostic utility of ChE activity in a field situation.