To investigate the protective effect of Gamdu-tang(GDT) and its constituents. Radix Glycyrrhizae(RG) and Semen Glycine(SG) on the damage of cardiac endothelial cells by xanthine oxidase (XO)/hypoxanthine (HX)-induced oxygen free radical, Neutral Red (NR) and DNA synthesis assay were used. The results were obtained as follows ; Cardiac endothelial cells treated with XO/HX showed the cytotoxicity such as decreases in viability and DNA synthesis. Cardiac endothelial cells pretreated with GDT extracts were not showed the decrease of DNA synthesis by XO/HX, These results show that XO/HX elicits toxic effects in cultured cardiac endothelial cells derived from neonatal rat, and suggest that GDT extract is very effective in the prevention of XO/HX-induced toxicity.