To test the protective effect of Sophorae Radix (SR) on the damage of cardiac endothelial cells by xanthine oxidase (XO)/hypoxanthine (HX)-induced oxygen tree radical, Neutral Red (NR), lactate dyhydrogenase (LDH), and c-fos immunopositive cells assay were used in the presence of SR extract. The results of these experiments were obtained as follows ; Cardiac endothelial cells treated with XO/HX showed the cytotoxicity such as a decrease in viability, and increases in LDH activity and c-fos immunopositive cells. Cardiac endothelial cells pretreated with SR extract protected the increase of LDH activity. Alos, cardiac endothelial cells pretreated with SR extract inhibited the increase of c-fos immunopositive cells. These results show that XO/HX induces toxic effects in cultured cardiac endothelial cells derived from neonatal rat, and suggest that SR extract is very effective in the prevention of XO/HX-induced toxicity.