To evaluate an effect of ethanol pretreatment on the liver xanthine oxidase(XO) activity, 0.25ml of xylene(50% in olive oil) per 100g body weight was daily given four days to the rats at 2hrs after aministration of ethanol each day, while each control group(ethanol, xylene, olive oli) was treated as the same dose described as above. The animals were sacrificed at 24hrs after last injection. Xylene-treated rats showed the more decreased activity of liver XO compared to the control. But the pretreatment of ethanol to the xylene-treated rats enhanced the liver XO activity. Furthermore, the xylene-treated rats led to more increased Vmax value in liver XO compared to the only xylene-treated rats. On the other hadn, hepatic aldehyde dehydrogenase activity was more decreased in xylene-treated rats pretreated with ethanol than in xylene-treated rats. And the intermediated xylene metabolites, methyl benzylalcohol or aldehyde inhibited the XO activity "in vitro". In conclusion, the phenomenon that pretreatment of ethanol to the xylene-treated rats led to the enhancement of liver XO activity, may be due to an influence of acetaldehyde.taldehyde.