Tri‐o‐cresyl phosphate (TOCP) and O‐ethyl O‐(4‐cyanophenyl) phenylphosphonothioate (cyanofenphos, Surecide) were found to be delayed neurotoxicants. They were administered to chickens by gavage at 100 and 30 mgjkg•d for 15 d, respectively. In CD‐1 mice neither TOCP nor cyanophenphos induced any of the usually recognized clinical symptoms of neuropathy when administered daily by gavage at 262 or 31.25 mg/kg‐d for 30 d, respectively. In the chickens, TOCP and cyanofenphos produced about 98 and 90% in vivo inhibition of brain neurotoxic esterase (NTE) activity. In the mice, 24 h after the last daily dose, TOCP and cyanofenphos produced only about 50 and 40% in vivo inhibition of the brain NTE activity. Parathion [O,O‐diethyl O‐(4‐nitrophenyl) phosphorothioate], at 2 or 6.75 mg/kg'd for 15 or 30 d, did not induce neuropathy in either chicken or mice and produced no significant in vivo inhibition of brain NTE activity at the end of the dosing regimen. The specific activity of NTE in control chicken brain crude homogenate was much higher than that in mouse brain homogenate. These results suggest that the differences between chickens and mice in susceptibility to neurotoxic organophosphates may be attributed to (1) inhibitor specificity of NTE forms in the brain in these two different animal species and/or (2) inability of the active metabolites of these neurotoxic compounds to reach the site of action.