A single i.p. administration of 1.0 mg/kg of chlorphenvinphos (CVP), an organophosphorus pesticide, results in an acute stress response, evidenced by a marked (6-7 fold) rise in plasma corticosterone (CORT) concentration, and a diminished behavioural sensitivity to amphetamine (AMPH) three weeks postexposure. Surprisingly, in rats subjected to a single series of inescapable electric footshocks (60 10 msec triplets of 3.0 mA, 2 msec, square pulses during 20 min - IF ) two weeks prior to the CVP exposure, these effects are not observed. It has been assumed that the reduced effectiveness of CVP might be related to some persisting alterations in the functional state of the cholinergic system. The aim of the present work was to discover whether and in what way the IF pretreatment affects i) the cholinesterase activity in blood, and ii) the dynamics of the alterations in the cholinesterase (ChE) activity following the CVP exposure. The experiments were performed on 3 mo. old, male Wistar rats. In the first experiment, the blood samples were taken from the tail vein 15, 60 and 180 min after the IF. In the second experiment, the rats were pretreated with IF and 14 days later given 1.0 mg/kg of CVP i.p. Blood samples were taken 15 min, 60 min, 180 min, 24 h, 7 days, and 14 days after the CVP exposure. In the first experiment no differences in the ChE activity in plasma (pChE) and erythrocytes (rbcChE) were found between the shocked and control rats. In the second experiment, however, in rats pretreated with IF the rbcChE activity of was reduced by CVP less and pChE activity returned to normal faster than in rats not pretreated with IF. The results confirm that exposure to IF, a nonchemical stressor, induces some long-lasting adaptive changes which render the cholinergic system less susceptible to the harmful action of ChE inhibitors. It has been hypothesized that the changes consist in an increase of the antioxidant potential in blood and possibly other tissues.