To investigate an effect of cyclohexanone (CHO) treatment on the serum levels of xanthine oxidase (XO) in liver damaged animals, the rats were intraperitoneally pretreated with 50% carbon tetrachloride ($CCl_4$) in olive oil (0.1 mL/ 100 g body weight) 14 times every other day. To the $CCl_4$-pretreated rats, CHO (1.56 g/kg body weight) was injected once and then the animals were sacrificed at 4 hours after CHO treatment. The increasing rate of serum and liver XO activities to the control was higher in CHO-treated animals pretreated with $CCl_4$ than the $CCl_4$-pretreated those. Concomitantly CHO injection to the $CCl_4$-pretreated animals showed somewhat higher Vmax and lower Km value in the kinetics of liver XO enzyme. Furthermore, increasing rate of hepatic malonedialdehyde content to the control was also higher in CHO-treated animals pretreated with $CCl_4$ than $CCl_4$-pretreated those. On the other hand, the injection of CHO to the $CCl_4$-pretreated animals showed the more enhanced liver damage on the basis of liver function finding; liver weight per body weight (%), serum levels of alanine aminotransferase activity and hepatic glucose-6-phosphatase activity. In conclusion, injection of CHO to the $CCl_4$-pretreated rats led to more increased activity of serum XO and it may be caused by acceleration of hepatocyte membrane permeability and induction of enzyme protein.