Male rats were given mercuric chloride (0.018 mmol/kg/day) by subcutaneous injection and zinc acetate (3.0 mmol/kg/day) by oral administration. Mercury and zinc were administered at the same time, once every 24 hr for 5 days. Only 1 of 10 rats given mercury alone survived for 3 days, and this one rat died on the 4th day. In the group given mercury and zinc at the same time, all the 10 animals were alive on the 5th day, indicating the marked effect of zinc in suppressing the toxicity of mercury. Based on such a marked effect of zinc, examinations were made to see whether biosynthesis of metallothionein would occur in the presence of zinc or mercury by the incorporation of ¹⁴C-cysteine into the metallothionein fraction. High rate of incorporation of radioactivity into the metallothionein in the rat liver was observed by the administration of zinc but the incorporation was not so marked by the administration of mercury. This fact suggested that the metallothionein induced by zinc might have some effect on lowering of mercury toxicity and then, distribution of mercury and zinc in the soluble fraction was examined. It was found that, in the liver, mercury bound in larger amount to high molecular fraction during single administration of mercury transited to the metallothionein fraction when zinc was administered concurrently. The amount of mercury bound as Hg-thionein in the kidney after the concurrent administration of mercury and zinc is about 4 times that after single administration of mercury.