Cholinesterase (ChE) activity in the blood serum of rats was elevated to 15, 25, and 45 times by the sc administration of 1000, 2000 and 3000 electric eel acetylcholinesterase (AChE) units respectively. Apparently no ill-effect to animals was observed. The maximal activity of the enzyme occurred in 90 min after its administration and was directly proportional to the administered dose. The increase activity of ChE in the serum on the exogenous administration of AChE persisted for 18 hr. The exogenously raised serum ChE, protected rats against lethal dose of dichlorvos, but not against lethal dose of soman. The possible mechanism of differential response in discussed.