The carcinogenic effectiveness of a number of nitrosoalkylamides related to nitrosoethylurea and nitroso-n-propylurea has been compared by topical application to Swiss mice and by intragastric administration to F344 rats. Nitrosohydroxyethylurea and the related cyclic nitrosamide, nitrosooxazolidone, were as potent as nitrosoethylurea as skin carcinogens, although the latter was a much weaker mutagen to Salmonella. When administered p.o., nitrosooxazolidone induced mainly forestomach tumors in rats, while nitrosohydroxyethylurea was very broadly acting, inducing neoplasms of the lung, forestomach, glandular stomach, colon, duodenum, and bone (osteogenic sarcomas). Nitroso-2-hydroxypropylurea, nitroso-3-hydroxypropylurea, and nitroso-5-methyloxazolidone were all much more potent carcinogens on mouse skin than was nitroso-n-propylurea, nitroso-5-methyloxazolidone being somewhat less effective than the nitrosoureas; the mutagenicity to Salmonella seemed not to be quantitatively related to carcinogenicity. Nitroso-5-methyloxazolidone given p.o. to rats induced mainly forestomach neoplasms and a few neoplasms of the duodenum, whereas similar treatment of rats with nitroso-2-hydroxypropylurea induced a high incidence of neoplasms of the thymus, some of the forestomach, and few at any other site.