[ ³ H]Cystine and ^115mCd were incorporated into hepatic and renal Cd‐thionein in response to sc administration of 4.4 μmol of Cd ²⁺ containing ^115mCd. Cd‐thioneinbound ^115mCd reached a plateau by 24 and 72 h after the Cd ²⁺ injection in liver and kidney, respectively. The half‐life (t _½ ) of ³ H‐labeled hepatic Cd‐thionein was 3.5 d, whereas the average t_½ of the soluble proteins was 3.7 d. The t_½ of ³ H‐labeled renal Cd‐thionein was 3.7 d, whereas the average t_½ of the soluble renal proteins was 3.8 d. In marked contrast, the ^115m Cd content of both hepatic and renal Cd‐thionein was virtually unchanged, even 9 d after administration of this radionuclide. These data indicate that the protein moiety of metallothionein is degraded, although there appears to be a concomitant rebinding of Cd ²⁺ to nascent thionein polypeptide chains. Thus the lack of metallothionein degradation per se does not account for the long‐term retention of Cd ²⁺ in liver and kidney during chronic exposure.