Depletion of serotonin or norepinephrine in male rats by pretreatment with p‐chlorophenylalanine or DL‐α‐methyl‐p‐tyrosine, respectively, failed to reduce the lethality of the monoamine oxidase‐inhibiting acaricide, chlordimeform. Administration of phenylephrine, a directly acting a‐adrenergic agonist, with chlordimeform did not increase the lethality of chlordimeform. Thus, monoamine oxidase inhibition by chlordimeform does not seem to contribute to its lethality.