Metallothionein (MT) is expressed to a certain extent in almost all mammalian tissues. The biological significance of MT is related to its various forms MT-1, MT-2, MT-3 and MT4. For MT-1 several isoforms of the protein exist and it is likely that these isoforms are related to various functions involved in developmental processes occurring at various stages of gestation. Toxicokinetics and biochemistry of essential and toxic metals such as cadmium, zinc, mercury and copper in organs e.g. kidney, CNS, are often related to metallothionein. It is debated whether there is a relation or not for other metals e.g. selenium and bismuth. For the toxicokinetics of cadmium, MT plays an important role. By expanding techniques from experimental toxicology and biochemistry to include molecular biology methods, more specific and relevant studies can be performed of the actual role and biological function of MT. The present paper on toxicological aspects of metallothionein, presents an overview and evaluation of present knowledge concerning differences among organs and within organs of the expression of MT and how this affects tissue sensitivity to toxicity.