Methyl methanesulphenate has been prepared labelled with radioactive carbon and its metabolism studied in rat, mouse and rabbit. The distribution of the drug after injection has been determined in the rat. Quantitative studies in this species have shown that less than as % of the injected drug is excreted or exhaled in 24 hours. Assay of tissue levels of radioactivity after injection has confirmed that the majority of the drug remains bound within the animal body. In the rat, the principal metabolic reaction has been shown to take place in the liver with glutathione. The S-methylglutathione formed is excreted in bile and this intermediate was the source of the urinary metabolites. In the urine, conjugates of S-methylcyateine and S-methylthioglycollic acid, together with small quantities of radioactive urea, have been recognized. The main metabolite has not been identified but appears to be a substituted guanidine compound. The possible significance of these results of these results has been discussed in terms of sell biochemistry.