For a large number of N-nitroso compounds a comparison of their carcinogenic effects in rats and Syrian golden hamsters has been made. Nitrosamines, which require metabolic activation, and nitrosoalkylamides, which do not, produce quite different tumor responses. There are also large differences in the types of tumor induced in rats and in hamsters. In all the studies doses of the various compounds, equimolar to the extent that was possible, are administered orally. Continuous doses (in drinking water or food) often produce a response different from that after administration of the same compound in pulsed doses (by gavage), even though the same total dose is delivered. Continuous doses of nitrosamines are usually more effective than pulsed doses, but with the nitrosoalkylureas, the reverse is more generally the case. Rat and hamster liver is a common target of many nitrosamines, but rarely of nitrosamides. The most common site of tumor induction in rats by N-nitroso compounds is the esophagus, but the hamster esophagus never responds. The pancreas duct of the hamster is a common target of nitrosamines containing a beta-oxygenated propyl group, but pancreas duct tumors are never seen in rats. Nitrosomethyl-n-alkylamines (with an even numbered carbon chain) induce bladder tumors in rats and hamsters. Many nitrosoalkylureas induce tumors of the nervous system in rats, as well as a great variety of other tumors. In hamsters, nitrosoalkylureas give rise only to tumors of the forestomach and spleen, but no tumors of the nervous system. The similar carcinogenic actions of certain groups of N-nitroso compounds can be related to their generation, directly or by metabolism, of similar simple moieties having certain organs as their target.