A number of reports indicate that exposure to organophosphates (OPs), inhibitors of acetylcholinesterase (AChE), may result in long-lasting neurobehavioural alterations suggestive of an increased cholinergic tone. It is known that rats with cholinergic hyperreactivity are behaviourally hyposensitive to cholinergic antagonists and dopaminergic agonists. The purpose of the present study was to find out whether a similar trait would develop in rats exposed to chlorphenvinphos (CVP), an OP pesticide, in the past. The rats were given ten daily i.p. injections of CVP at doses of 0.5 mg/kg (group P-0.5) or 1.0 mg/kg (group P-1.0). The locomotion stimulating effect of i.p. injection of 1.0 mg/kg amphetamine (AMPH), or 0.7 mg/kg scopolamine (SCOP), was assessed on postexposure day 21 (group P-0.5) or 42 (group P-1.0), i.e. after a time sufficient for AChE recovery. The assessment revealed that in group P-1.0 the behavioural response to AMPH and SCOP was significantly depressed. In rats of the P-0.5 group, however, the behavioural response to each of the drugs was increased. The results suggest that, depending on the exposure level, contrasting alterations in some neurotransmitter systems may be induced by repeated exposure to CVP.