Metallothionein (MT) mRNA expression was investigated in a rodent model (G93A SOD1 transgenic mouse) for a lethal motor neuron disease, amyotrophic lateral sclerosis (ALS). In 8-wk-old mice that did not yet exhibit motor paralysis, MT-I mRNA expression was already significantly upregulated in the region of the spinal cord responsible for motor paralysis. The expression of another isoform, MT-III, was not changed. In the cerebellum, which is not responsible for motor paralysis in ALS, neither the expression profiles of MT-I nor MT-III were altered. In 16-wk-old mice exhibiting motor paralysis, the expression of MT-I mRNA remained upregulated and the MT-III level tended to be elevated. Although no significant differences were found in the levels of both isoforms in the liver or kidney of 8-wk-old mice, the MT-I mRNA expression level was significantly upregulated in the kidney and liver of 16-wk-old mice. These results indicated that the MT-I isoform, but not the MT-III isoform, is associated with motor neuron death in ALS and suggested that the disease might be a systemic disorder to which the spinal cord is particularly susceptible.