Metallothionein is an integral component of the mechanism that regulates the metabolism of cadmium and zinc. The synthesis of this protein can be "induced" by oral or parenteral administration of either metal. The metallothionein mRNA content of liver polysomes is increased shortly after an influx of small amounts of either metal into hepatocytes. After sufficient amounts of this poly (A+) RNA have been synthesized, there is a concomitant increase in metallothionein biosynthesis and metal binding. Unlike synthesis, the degradation of metallothionein is markedly influenced by the species of metal bound. By using in vivo and in vitro techniques, it has been possible to demonstrate that resistance of metallothionein to degradation follows the order: thionein less than zinc metallothionein less than cadmium metallothionein. Moreover, while the polypeptide chains of cadmium metallothionein are degraded, it appears that liberated cadmium ions are quickly incorporated into nascent chains of thionein. The latter explains why the cadmium content of liver and kidney increases with age and environmental exposure. Since both zinc and cadmium bind to metallothionein, it appears that the binding sites provided by this inducible species provide a locus for interaction between zinc, a nutrient, and cadmium, an environmental contaminant.