Newborn rats have about a 20-fold higher concentration of metallothionein in the liver than does the adult rat. This marked increase in hepatic metallothionein does not result in a lower acute toxicity to cadmium in newborn rats; it does not markedly increase the distribution of cadmium to the liver, decrease its concentration in other tissues, or produce a slower removal of the cadmium from younger animals. The toxicity of cadmium in the newborn rat appears to be more dependent on other factors such as the development of the blood–brain barrier and the stage of development of the testes than on the amount of metallothionein in the liver. The results of this study suggest that we should probably reevaluate the role of metallothionein in producing tolerance to cadmium.