Five cyclic nitrosamines, four containing oxygen in the ring, were administered by gavage to groups of 20 male Syrian golden hamsters. After administration of very similar doses, nitrosomorpholine, nitroso-2-methylmorpholine and nitroso-5-methyl-1,3-oxazolidine caused the animals to die with tumors after similar times, but nitrosomorpholine induced mainly tumors of the nasal cavity (and a few of the trachea), whereas the 2-methyl derivative induced tumors of the nasal cavity and liver. While nitroso-1,3-oxazolidine and its 5-methyl derivative both induced liver tumors (but no tumors in the nasal cavity) those induced by the former compound took much longer to kill the animals. Nitrosoazetidine, a liver carcinogen in rats, but which had been reported to be inactive in hamsters, did induce tumors of the liver in 30% of hamsters after a much larger dose than the other cyclic nitrosamines.