To establish for the first time how mice might differ from rats and humans in terms of copper transport, excretion, and copper binding proteins, plasma and organ cytosols from adult female C57CL6 mice were fractionated and analyzed by directly coupled size exclusion HPLC-ICP-MS, before and after i.p. injection of large doses of (65)Cu. Plasma from untreated mice had different proportions of Cu associated with transcuprein/macroglobulin, ceruloplasmin and albumin than in humans and rats, and two previously undetected copper peaks (Mr 700 k and 15 k) were observed. Cytosols had Cu peaks seen previously in rat liver (Mr > 1,000 k, 45 k and 11 k) plus one of 110 kDa. (65)Cu (141 microg) administered over 14 h, initially loaded plasma albumin and mainly entered liver and kidney (especially 28 kDa and 11 kDa components). Components of other organs were less (but still significantly) enriched. (63)Cu/(65)Cu ratios returned almost to normal by 14 days, indicating a robust system for excreting excess copper. We conclude that there are significant differences but also strong similarities in copper metabolism between mice, rats and humans; that the liver is able to buffer enormous changes in copper status; and that a large number of mammalian copper proteins remain to be identified.