To certify the protective effect of herbal medicine against oxygen free radical-induced myocardiotoxicity, cytotoxicity was measured using MTT, LDH activity and Beating rate assay in the presence of Guaruhaebaekbanha-tang(GHBT) extracts or single constituents of this prescription. Myocardial toxicity was evaluated in neonatal rat myocardiocytes in cultures. In the present study, xanthine oxidase/hypoxanthine(XO/HX) resulted in a decrease in cell viability, increases in LDH activity in culture medium and decreases in beating rate in cultured myocardial cells. In the effect of GHBT extract, it showed the prevention from the XO/HX-induced cardiotoxicity by the increases of cell viability and beating rate as well as the decrease of LDH activity. In the protective effect of Fructus Trichosanthis(FT), Bulbus Allii Macrostemi(BAM) and Rhizoma Pinelliae(RP), all the extracts were significantly effective in the protection of XO/HX-induced cardiotoxocity in cultured myocardial cells by the increase of beating rate as well as th decrease of LDH activity. From these results, they show that XO/HX is cardiotoxic in cultured myocardial cells derived from neonatal rat, and it suggests that GHBT, FT, SAM, RP extracts are positively effective in the blocking in XO/HX-induced cardiotoxicity.