Alkanediazohydroxides are common key intermediates in carcinogenesis and mutagenesis of N-nitroso compounds, which are widely found in human environment. Mutagenicity of (E)- and (Z)-potassium alkanediazotates, as precursors of corresponding alkanediazohydroxides were evaluated to investigate the effect of geometric isomerism and also the effect of alkyl groups on their biological activity. Mutagenicity of N-nitroso-N-alkylureas which spontaneously produce alkanediazohydroxides after non-enzymatic hydrolysis were also tested in comparison to that of the corresponding diazotates and other activated chemical species of N-nitrosamines. When the mutagenicity was assayed in three microbial strains, Salmonella typhimurium TA1535, and Escherichia coli WP2 and WP2 uvrA, the order of mutagenic potency of the compounds with the same alkyl group was as follows; (E)-diazotates > (Z)-diazotates > nitrosoureas. The effect of alkyl groups on the mutagenic potency was different in Salmonella strain and in E. coli strains, and this result could be explained by the efficiency of O6-alkylguanine-DNA alkyltransferase. In each bacterial strain, this effect of alkyl groups was similar in mutagenicity induced by (E)- and (Z)-diazotates, N-nitroso-N-alkylureas and other activated N-nitrosodialkylamines such as alpha-hydroxy nitrosamines. The geometrical isomerism affected the mutagenicity of (E)- and (Z)-potassium alkanediazotates, and the result suggested that alkanediazohydroxides react through diazonium ions in a cage rather than through free alkyldiazonium ions which have no geometrical isomerism. Our results confirmed that (E)-potassium alkanediazotates, (Z)-potassium alkanediazotates and N-nitroso-N-alkylureas all decomposed through diazohydroxides, and that alkanediazohydroxides are the active alkylating species of N-nitroso compounds, and also that the geometrical isomerism is important for carcinogenic N-nitroso compounds to show their biological activity.