The effect of mercury and combined effect of mercury and DMSO on the activity of acetylcholinesterase of rat lymphocytes was studied during in vitro incubation. It was found that mercury is a very strong inhibitor of AchE. Dimethylsulphoxide performs a protective role for this enzyme. The activity of AchE in the lymphocytes treated with mercury and DMSO regardless of mercury concentration was similar to that in control series.