The mechanism of protective effect of sodium selenite (Se) on the nephrotoxicity of cis-diamminedichloroplatinum (CDDP) was studied with mice. CDDP and Se concentrations in the plasma, blood cell, kidney and liver were measured in mice given CDDP alone and those given both CDDP and Se. The nephrotoxicity of CDDP was observed in mice fed Se deficient diet (SDD). The activities of glutathione peroxidase (G-Px) in the plasma, blood cells, kidney and liver were measured in mice mentioned above. Co-administration of Se did not influence on the concentration of CDDP in the blood, kidney and liver. In mice fed SDD, the nephrotoxicity of CDDP increased and G-Px activities of the blood and kidney decreased. In mice given Se, G-Px activities was not increased. These results suggest that interaction between CDDP and Se differs from that between mercury and Se, or between cadmium and Se (formation of compound). Intake of Se is related to the appearance of the nephrotoxicity of CDDP. G-Px may be related partially to the protective effect of Se on the nephrotoxicity of CDDP.