SePP (selenoprotein P) is central for selenium transport and distribution. Targeted inactivation of the Sepp gene in mice leads to reduced selenium content in plasma, kidney, testis and brain. Accordingly, activities of selenoenzymes are reduced in Sepp(-/-) organs. Male Sepp(-/-) mice are infertile. Unlike selenium deficiency, Sepp deficiency leads to neurological impairment with ataxia and seizures. Hepatocyte-specific inactivation of selenoprotein biosynthesis reduces plasma and kidney selenium levels similarly to Sepp(-/-) mice, but does not result in neurological impairment, suggesting a physiological role of locally expressed SePP in the brain. In an attempt to define the role of liver-derived circulating SePP in contrast with locally expressed SePP, we generated Sepp(-/-) mice with transgenic expression of human SePP under control of a hepatocyte-specific transthyretin promoter. Secreted human SePP was immunologically detectable in serum from SEPP1-transgenic mice. Selenium content and selenoenzyme activities in serum, kidney, testis and brain of Sepp(-/-;SEPP1) (SEPP1-transgenic Sepp(-/-)) mice were increased compared with Sepp(-/-) controls. When a selenium-adequate diet (0.16-0.2 mg/kg of body weight) was fed to the mice, liver-specific expression of SEPP1 rescued the neurological defects of Sepp(-/-) mice and rendered Sepp(-/-) males fertile. When fed on a low-selenium diet (0.06 mg/kg of body weight), Sepp(-/-;SEPP1) mice survived 4 weeks longer than Sepp(-/-) mice, but ultimately developed the neurodegenerative phenotype. These results indicate that plasma SePP derived from hepatocytes is the main transport form of selenium supporting the kidney, testis and brain. Nevertheless, local Sepp expression is required to maintain selenium content in selenium-privileged tissues such as brain and testis during dietary selenium restriction.