AbstractThe synthesis of14C and3H labelled methylazoxymethylacetate, ( MAM‐ O Ac), a hepatotoxic and carcinogenic compound, was accomplished by the incorporation of a radioisotope labelled methyl group into N,N′ ‐dimethylhydrazine with methyl iodide instead of dimethyl sulfate. Monosodium dibenzoylhydrazine was methylated with labelled methyl iodide and the second methyl group was subsequently added with unlabelled dimethyl sulfate to produce N,N′ ‐dimethylhydrazine. This modification made it possible to prepare labelled N,N′ ‐dimethylhydrazine with a small amount of total radioactivity and avoid an excessive waste of the methyl label. The labelled compound was then used in the synthesis of MAM‐OAc by a previously published procedure. The incorporation of radioactivity into N,N′‐dimethylhydrazine was in the range of 75‐95 %. Better yields were obtained for3