Metallothionein (MT) gene expression in the brain has been most thoroughly studied using rodents. Although MT is considered to be a 'housekeeping' protein even in the brain, the basal MT mRNA expression level is not always high. Differences in the responses of rats and mice have made it difficult to interpret the data. Moreover, the response to inducers is not always apparent, probably because the brain is protected by the blood-brain barrier and initial responses to inducers in peripheral tissues modulate their accumulation in the brain. A relatively high content of MT protein in the brain might be sufficient to elicit minute alterations in the level of inducers. Nonetheless, regulation of MT gene expression in the brain seems to be important in e.g. maintaining the levels of trace elements and controlling redox potentials. The localization and utilization of trans elements such as MTF-I and MEP-I in the brain will provide new aspects for study. The high homology among MT isoforms with respect to nucleotide as well as amino acid sequences has made it difficult to obtain cDNA probes or antibodies capable of distinguishing MT isoforms. Thus, their cross-reactivity might make changes in MT mRNAs appear minimal when MT isoforms are differently regulated. The rapid developments in methodology permitting sensitive, rapid, high-resolution analysis could clarify the background of tissue- and cell-specific gene regulation as well as differential induction.