Chlorpyrifos is widely used to control agricultural pests associated with fruit, nut and vegetable crops, despite its toxic effects and potential brain alterations in aquatic organisms. This study was carried out to determine the in vivo and in vitro effects of chlorpyrifos on rainbow trout brain acetylcholinesterase (AChE) enzyme activity. The fish were exposed to 2.25 mu g/L (25% of 96 h LC50), 4.5 mu g/L (50% of 96 h LC50) and 6.75 mu g/L (75% of 96 h LC50) of chlorpyrifos for 24, 48, 72 and 96 h. In vitro studies, inhibition constants (K-i) and half-maximal inhibitory concentration (IC50) values for chlorpyrifos were determined by Lineweaver-Burk graphs and by plotting activity percentage vs. [I], respectively. Enzyme activities were determined by a colorimetric method. In fish exposed to chlorpyrifos 2.25 and 4.5 mu g/L, enzyme inhibition was not observed (p > 0.05); however, chlorpyrifos exposure to 6.75 mu g/L for 72 and 96 h caused a significant decrease in enzyme activity (p < 0.05). The IC50 value of chlorpyrifos was found to be 30 mu g/L. These results show that chlorpyrifos may cause direct cellular injury in the brain, and suggest that AChE may be used as a bioindicator for toxicological studies.