Male rats were given subcutaneous injection of mercuric chloride (0.018 mmole/kg/day) and oral administration of zinc acetate (3.0 mmoles/kg/day). 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 4 th day. In the animals given mercury and zinc at the same time, all the 10 animals were alive on the 5 th 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 by the presence of zinc or mercury from 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 seems to suggest that the biosynthesis of metallothionein by zinc is responsible for the suppressive effect of zinc on the toxicity of mercury.