The renal uptake and degradation of cadmium-thionein (Cd-Mt) were examined in relation to nephrotoxic effects. Studies with Cd-Mt labelled with [3H]cystine showed that both Cd2+ and tritium uptake in the kidneys were complete 4 h after injection. During this period, renal copper content doubles due to the replacement of thionein-bound Cd2+ with Cu2+. This process probably occurs in the blood, prior to metallothionein uptake. Once reabsorbed, the protein is rapidly degraded in the lysosomes at a rate in step with uptake. Consequently, at 4 h virtually all of the Cd-Mt was degraded, resulting in a high concentration of non-thionein bound Cd2+. This Cd2+ (approx. 11-12 micrograms Cd2+/g, i.e. 70% of the total renal Cd2+ burden) produces the toxic effects. Between 2-4 h, new thionein synthesis is initiated and Cd2+ gradually becomes bound as the metallothionein. By 4 days, 80% of the renal Cd2+ is bound to endogenous thionein. These studies demonstrate that even small amounts of non-thionein bound Cd2+ are toxic to the kidney.