This study was planned to reveal in vivo genotoxic, antigenotoxic and cytotoxic properties of methanolic extracts of St. John's Wort plant (Hypericum perforatum). The methanol extract of H. perforatum was administrated to rats alone or in combination with N-nitrosodimethylamine and Triclosan (positive control). The concentrations of H. Perforatum were 20, 100, 500 mg/kg and the period of administration was 30 days. At the end of this period, chromosome aberration test used to determine genotoxic and antigenotoxic effects of H. perforatum. In addition, total oxidant and antioxidant levels of peripheral blood serum of rats were determined by spectrophotometric method. According to our result, H. perforatum extract did not demonstrate genotoxic effect at concentrations administrated. When compared to the negative control, the percentage of abnormal cells was increased in groups treated with test substance, N-nitrosodiethylamine and triclosan together. However, these increases were not as higher as groups treated with N-nitrosodiethylamine plus triclosan (positive control). Even, in groups treated with the test substance additional N-nitrosodiethylamine plus triclosan, the ratio of abnormal cells was inversely proportional to the increase in the concentration. The lowest ratio of abnormal cells was in the group in which the highest concentration of the H. perforatum (500 mg/kg) was applied. The highest two concentrations of the H. perforatum (100 and 500 mg/kg) significantly increased the mitotic index. In the group treated with test substance and positive control together, the mitotic index was even greater. At the same time 100 mg/kg of H. perforatum extract significantly reduced the oxidative stress.