Deletion of the mouse selenoprotein P gene (Sepp1) lowers selenium concentrations in many tissues. We examined selenium homeostasis in Sepp1^−/− and Sepp1^+/+ mice to assess the mechanism of this. The liver produces and exports selenoprotein P, which transports selenium to peripheral tissues, and urinary selenium metabolites, which regulate whole-body selenium. At intakes of selenium near the nutritional requirement, Sepp1^−/− mice had whole-body selenium concentrations 72 to 75% of Sepp1^+/+ mice. Genotype did not affect dietary intake of selenium. Sepp1^−/− mice excreted in their urine approximately 1.5 times more selenium in relation to their whole-body selenium than did Sepp1^+/+ mice. In addition, Sepp1^−/− mice gavaged with ⁷⁵SeO₃²⁻ excreted 1.7 to 2.4 times as much of the ⁷⁵Se in the urine as did Sepp1^+/+ mice. These findings demonstrate that deletion of selenoprotein P raises urinary excretion of selenium. When urinary small-molecule ⁷⁵Se was injected intravenously into mice, over 90% of the ⁷⁵Se appeared in the urine within 24 h, regardless of selenium status. This shows that urinary selenium is dedicated to excretion and not to utilization by tissues. Our results indicate that deletion of selenoprotein P leads to increased urinary selenium excretion. We propose that the absence of selenoprotein P synthesis in the liver makes more selenium available for urinary metabolite synthesis, increasing loss of selenium from the organism and causing the decrease in whole-body selenium and some of the decreases observed in tissues of Sepp1^−/− mice.