Measurement of the activity of the serum acetylcholinesterase (AChE) enzyme provides valuable data about the overexposure of AChE-inhibiting pesticides such as the organosphosphate (OP) insecticides. Because these compounds are efficiently absorbed by oral, dermal, conjunctival and respiratory routes, agricultural workers are at high risk of acute or chronic OP poisoning. The goal of the present study was to examine the effects of the OP compounds on serum AChE activity in agricultural workers who have been repeatedly exposed to pesticides for several years. Thirty pesticides sprayers (ages 21 to 49) and 30 healthy volunteers (ages 22 to 65) not exposed to pesticides were enrolled to the study. Workers were also divided into separate groups according to their exposure years and smoking habits. Blood samples were obtained by venipuncture one day after the spraying season. AChE analyses were performed using a colorimetric method and commercial kits (SIGMA, USA). Normal range of this method was 40-120 Rappoport Units (RU). Data were analyzed by the Student's t test and the regression analysis. Mean serum AChE levels in agricultural workers and control group were 54.9 [plusmn] 3.3 RU (range from 26 to 102) and 64.9 [plusmn] 2.7 RU (range from 46 to 105) respectively. In agricultural workers, AChE activity was significantly lower than the control group, p [lt ] 0.05). There was no correlation between the number of working years and AChE activity (r = [minus ]0.03). Similarly smoking habit had no significant effect on the serum AChE levels (p [gt ] 0.05). The workers were using gloves and face masks as safety measures. We conclude that repeated exposure to OP compounds increases the risks of pesticide poisoning and performed safety measures are not efficient for preventing the toxicity. OP exposure may be minimized by optimizing working conditions and education programmes.