From April 1997 to June 1998, 14 measurements of acetylcholinesterase (AChE) enzymatic activity were performed with the copepod, Tigriopus brevicornis, collected at five stations in the Vilaine River estuary (South Brittany, France). Simultaneously, four chemical analyses of triazines and one analysis of total pesticides in water were undertaken. AChE activity levels in T brevicornis were compared to the levels measured at a reference site not exposed to effluents from Vilaine River. Results reveal significant differences between AChE activity levels depending on location of stations in the plume of the river with an increasing gradient of activity from the upstream to the downstream stations, thus indicating that neurotoxic contaminants are mainly brought by the river. The average degree of AChE inhibition between the reference site and the most upstream site is 70-80% during spring in 1997 and 1998. In May 1997, live copepods from the different sites were brought back and transferred to clean seawater. After 14 days, recovery of AChE activity was almost total when compared to the control. Moreover, using a linear regression model and the atrazine concentration as marker of the presence of pesticides, low levels of AChE activity were significantly explained by atrazine concentration in Water.