The kinetic distribution patterns of [64Cu]Cu2+ in normal mice and in mice bearing tumors (HepA, ascitic tumor) after i.v. injection were similar. The i.v. injected [64Cu]Cu2+ concentrated into mouse liver first and then went to other organs and tissues, such as kidney and tumor. Most of the [64Cu]Cu2+ injected concentrated into the liver within 4 h, and about 8% of the total [64Cu]Cu2+ injected concentrated into the tumor cells at 24 h after i.v. injection. About 80% and 90% of the soluble [64Cu]Cu in the livers of tumor-bearing mice and normal mice, respectively, existed as [64Cu]CuMT (metallothionein [MT] is a small protein with many cysteine residues) at 4 h after i.v. injection, while about 43% of the soluble [64Cu]Cu2+ in tumor cells combined with MT at 6 h after i.v. injection. After 10 days oral administration of 150 microg/g body weight copper acetate, the concentration of MT in tumor cells reduced sharply, from 316 microg/g tissue to 152 microg/g tissue, while it increased slightly, from 375 microg/g tissue to 439 microg/g tissue, in the livers of tumor-bearing mice (HepA, solid tumor). The results suggest that MT contributes to the metabolism of copper that is localized mainly in the liver after copper administration and that copper can concentrate into mouse tumor cells followed by the reduction of MT in tumor cells.