Three nitrosoalkylureas, two nitrosotrialkylureas, and three nitrosoalkylcarbamates were given to Syrian golden hamsters by gavage at approximately equimolar doses. Measured by the time to death with tumors as an index, nitrosoethylurea was the most potent carcinogen, followed by nitroso-2-hydroxyethylurea, which was less effective in males than in females. The least effective compounds, by this measure, were nitrosooxazolidone and nitroso-5-methyloxazolidone. The remaining compounds, nitroso-N-ethylurethan, nitroso-2-hydroxypropylurea, nitrosomethyldiethylurea, and nitrosotriethylurea appeared to be of similar potency. All of the compounds induced papillomas or carcinomas of the nonglandular stomach in high incidence, except in the groups given nitrosohydroxyethylurea or nitrosooxazolidone; exceptionally, only 35% of the latter group had tumors, compared with 70% or more in the other groups. All of the nitrosoalkylureas induced a high incidence of hemangiosarcomas of the spleen, but the nitrosoalkylcarbamates did not. The quite uniform response of the hamster to these compounds contrasts with the great variety of organs and cell types in which they induce tumors in the rat.