It has been studied if radical oxygen involves in the formation of a hemorrhagic inflammation in the testis of rats intoxicated with CdCl2. H2O2 production in rats testis increased significantly at 24 hrs after CdCl2 injection (5.0 mg/kg) and it reached the highest value at 48 hrs. The curve of H2O2 production was very similar to that of heme protein contents in the testis with time course. Enzyme activities of catalase, glutathione peroxidase and glutathione reductase increased simultaneously, whereas superoxide dismutase activity was convertly depressed in the testis of intoxicated rats. On the other hand, in the liver where no inflammatory change manifested in the testis was observed, there was no increase of radical oxygen production and the activity of catalase rather decreased. These results indicate that CdCl2 administration increases the formation of radical oxygen, and it followed by the occurrence of tissue damage (lipid peroxidation and denaturalization of protein) . The difference in the occurrence of radical oxygen between liver and testis may have a significant meaning for interpreting the tissue difference against the injury of CdCl2.