Objectives: The determination of cholinesterase activity has been commonly applied in the biomonitoring of exposure to organophosphates and carbamates and in the diagnosis of poisoning with anticholinesterase compounds. One of the groups who are at risk of pesticide intoxication are the workers engaged in the production of these chemicals. Aims: The aim of this study was to assess the effect of pesticides on erythrocyte and serum cholinesterase activity in workers occupationally exposed to these chemicals. Methods: The subjects were 63 workers at a pesticide plant. Blood samples were collected before they were employed (phase I) and after 3 months of working in the plant (phase II). Cholinesterase level in erythrocytes (EChE) was determined using the modified Ellman method, and serum cholinesterase (SChE) by butyrylthiocholine substrate assay. Results: The mean EChE levels were 48±11 IU/g Hb in phase I and 37±17 IU/g Hb in phase II (paired t-test, mean = -29; 95% CI = -43-14), p < 0.001). The mean SChE level was 9569±2496 IU/l in phase I, and 7970±2067 IU/l in phase II (paired t-test, mean = 1599; 95% CI = 1140-2058, p < 0.001). There was a significant increase in ALT level (p < 0.001) and a decrease in serum albumin level (p < 0.001). Conclusion: In view of the significant decrease in EChE and SChE levels among pesticide workers, it seems that routine assessment of cholinesterase level in workers employed in such occupations and people handling pesticides should be made obligatory.