A mutant strain (LEC) of rats was found to possess the feature of low degree of the zinc-induced hepatic metallothionein (MT) gene expression due to an alteration of the transcription factor concerned in the gene expression. Northern blot analyses showed that the amount of MT-1 mRNA induced by intraperitoneal zinc injection is smaller in LEC mutant rat liver than in normal rat liver, while the amount of MT-1 mRNA induced by copper injection is indistinguishable between LEC and normal rat livers. Gel retardation assays showed that LEC and normal rat livers are different in the nuclear protein which binds to the metal-responsive element (MRE) of the MT gene in a zinc-dependent manner, and that the efficiency of the zinc-dependent binding of the nuclear protein to the MRE is lower in LEC rat liver than in normal rat liver. LEC rat should provide a useful model to understand the transcription factor concerned in the MT gene expression by zinc.