The metabolic conversion of N-nitrodimethylamine and of N-nitrosodimethylamine was compared in vitro. The biochemical properties of the two compounds were nearly identical; however, the biological activities (carcinogenicity, mutagenicity and toxicity) of the nitramine are many times less potent. N-Nitrodimethylamine was found to be mutagenic to Salmonella typhimurium TA100 when applied at above 200 mumol/plate with metabolic activation. Its suggested metabolite, N-nitromethylamine, was not mutagenic, N-Nitromethylhydroxymethylamine, N-nitromethylacetoxymethylamine and formaldehyde were mutagenic only to S. typhimurium TA100 at low concentrations and toxic above 2 mumol/ plate. The evidence suggests that formaldehyde is the intermediate responsible for the mutagenicity of the nitramine derivatives and of the parent compound, N-nitrodimethylamine.