Global cerebral ischemia and reperfusion (I/R) often result in high mortality. Free radicals play an important role in global cerebral I/R. Xanthine oxidoreductase (XOR) inhibitors, such as allopurinol, have been reported to protect tissues from damage caused by reactive oxygen species (ROS) by inhibiting its production through XOR inhibition. The recently introduced XOR inhibitor febuxostat, which is a more potent inhibitor than allopurinol, is expected to decrease free radical production more effectively. Here, we analyzed the effects of allopurinol and febuxostat in decreasing global severe cerebral I/R damage in mice. Mice were divided into three groups: a placebo group, an allopurinol group, and a febuxostat group. Pathological examinations, which were performed in each group in the CA1 and CA2 regions of the hippocampus 4 days after I/R surgery, revealed that there was a decrease in the number of neuronal cells in the 14-min occlusion model in both regions and that drugs that were administered to prevent this damage were not effective. The enzymatic activity was extremely low in the mouse brain, and XOR could not be detected in the nonischemic and ischemic mice brains with western blot analyses. Thus, one of the reasons for the decreased effectiveness of XOR inhibitors in controlling severe whole-brain ischemia in a mouse model was the low levels of expression of XOR in the mouse brain.