The aim was to study the effects of dimethoate on enzymatic targets and on the growth of Helix aspersa for different times and modes of exposure under laboratory conditions. Young snails were exposed to increasing dimethoate concentrations in the food (D. exp) or in an artificial substrate (S. exp) for 1, 2, 7 and 14 days. Both acetylcholinesterase (AChE) and carboxylesterase (CaE) activities were measured in the foot of the snails for each concentration and exposure time tested. Growth was evaluated after 7 days of exposure. AChE inhibition, dose-dependent for all lengths of exposure, was stronger in S. exp. AChE was more sensitive than CaE for both modes of exposure. IC50-7 days was 38.3 mug g(1) in D. exp and 11.7 mug g(1) in S. exp for AChE and was higher than 150 mug g(1) in two exposure modes for CaE. AChE activity decreased from the first day to reach maximum inhibition after 7 days of exposure. As noted for B-esterase activities, growth inhibition was stronger in S. exp and was only significant for AChE inhibition of >90%. The present results show that AChE activity could be used to give early warning of toxic effects of dimethoate in terrestrial gastropods.