To examine the cytotoxicity of reactive oxygen species in cultured rat myocardial cells, cytotoxic effect was determined by MTT assay after cultured cells were incubated for 4 hours in the media containing 1～30μM of H₂O₂. And also, the protective effect of Drynariae Rhizoma(DR) was determined in these cultrures. Cell viability was significantly decreased in a dose-dependent manner after exposure of 15 μM H₂O₂ to cultured rat myocardials for 4 hours. In the protective effect of DR, DR prevented the H₂O₂-induced cytotoxicity in these cultures. From these results, it suggests that H₂O₂ has toxic effect in cultured mouse myocardial cells and DR has protective effect on the cytotoxicity induced by H₂O₂.