Methyl mercaptan is purchased by the Orbanic Chemicals Department for use in the manufacture of DL-methionine. The precautionary measures employed in this process, in addition to the warning provided by the extreme odor of the methyl mereaptan may be sufficient to preclude serious acute poisoning. However, there is very little quantitative information in the literature on the acute or subacute toxicity of methyl merceptan, and it has generally been assumed that the material is similar in toxicity to hydrogen sulfide. In the absence of such quantitative information on the toxicity of methyl mercaptan, the Haskell Laboratory was requested to make a preliminary acute and subacute inhalation toxicity study.