Metallothionein (MT) induction in the rat kidneys by endotoxin and zinc was investigated at the MT-protein level and the MT-mRNA level. The MT-mRNA level increased 3-6 hr after endotoxin administration, which suggested that MT was not transported from an other organ, but was de novo synthesized. Two MT-protein isoforms, MT-Ⅰ and MT-Ⅱ, were induced by administration of both endotoxin and zinc. The maximum levels of the proteins were seen 9 hr after administration of endotoxin and zinc. The span life of the renal MT induced by zinc was shorter than that of the liver MT induced by zinc. The MT-Ⅱ isoform was predominantly induced by both inducers, endotoxin and zinc, and the ratios of MT-Ⅱ to MT-Ⅰ were relatively constant. The finding that MT-Ⅰ and MT-Ⅱ were almost equally induced by zinc in the liver indicate that MT induction is controlled by an organ-specific system.