Abstract: In the present paper, Mestinon was tested for its possible genotoxicity by using sister chromatid exchange (SCE) as an indicator in human lymphocytes and cultured hamster Wg3-h cells. Experinental results indicated that the SCE frequency in human lymphocytes was incresed by Mestinon which is statistically significant(P 0.01 T test)at a dosage of 500μg/ml. The SCE rates of Wg3-h cells was also increased when Mestinon was actived by S-9 (The rat liver microsome preparation). The differences between the experimental data and the control were signifcant(P 0.001, T test).