The differential inhibition of acetylcholinesterase (AChE) by organophosphate (OP) and carbamate (C) is followed by the distinct duration of exposure effect on common carp AChE. Hence, in the present study in vivo exposure period effect and in vitro concentration-response of chlorfenvinphos, chlorpyrifos diazinon, and carbofuran were investigated on Cyprinus carpio L. AChE. Individuals of 1-year-old carp were exposed for 96 h to different concentrations of insecticides, after which AChE activity was measured in the brain. The highest concentrations of carbofuran (2.44 mg x L(-1)), chlorfenvinphos (2.9 mg x L(-1)), and diazinon (2.5 mg x L(-1)) killed all the test animals after only 4 h, although there was no statistically significant difference from the control group's brain AChE activity. The lowest concentration significantly inhibited brain AChE after 96 h. Chlorfenvinphos was the most potent inhibitor in vivo and chlorpyrifos the least active inhibitor after 96 h of exposure time. In vitro experimentation with the same pesticide indicated that several concentrations inhibited 50% of the AChE activity (I50), ranging from 4.1x10(-7) to 8.12x10(-4) M in both single inhibitory action and joint inhibitory effect. The results suggest that in biomonitoring programs carp brain AChE can be a good diagnostic tool for chronic OP nd C pollution.