The carcinogenic activities in rats and hamsters and the mutagenic activity in Salmonella of a number of N-nitroso compounds belonging to various classes have been compared. While most directly acting N-nitroso compounds and those requiring metabolic activation are mutagenic with appropriate activation and seem to alkylate DNA in vivo, there are exceptions. Some of these are mutagens that are not carcinogenic; others are carcinogens that are nonmutagenic. Even among the mutagenic carcinogens, there is no quantitative relationship between mutagenic and carcinogenic activities. This implies to directly acting compounds and to those requiring metabolic activation. The lack of congruence between the two activities among the nitrosamines is due to the complexity of the metabolic activating processes leading to formation of proximate carcinogens. The deficiencies in the mutagenesis assay appear to arise from a lack of the necessary enzymes in the liver microsomal fractions used for activation. Nitrosamines bearing oxygen on the beta carbon of an alkyl chain are not oxidized by rat microsomal enzymes and hence are not converted to bacterial mutagens by rat liver microsomes. Bacterial mutagenicity is not a guide to carcinogenic activity of N-nitroso compounds or to the mechanisms by which these compounds induce cancer.