To evaluate the postmortem changes in activities of oxygen free radical metabolizing enzymes, the rats were sacrificed with cervical dislocation and were kept in an incubator at $25^{circ}C$, 70% of humidity for 12 hours. The activities of aniline hydroxylase, catalase, glutathione-S-transferase and superoxlde dismutase were decreased with the time. On the other hand, the activity and type conversion ratio (type D ${ o}$type O) of hepatic xanthine oxidase (XO) were gradually increased. From these changes of XO, the estimation of death time (mathematical equation) could be determined with the least square method. To clarify the cause of increasing XO activity, enzyme kinetics were examined. The Km values of XO were decreased with the time. In conclusion, the determination of liver XO activity might be used for the estimation of death time in the early postmortem period.