Formation of N-nitroso compounds from amines and nitrite has been demonstrated in chemical systems and in the stomach of animals in vivo. The amines differ in the extent to which they give rise to N-nitroso compounds. Many amines are common in the environment. Other amines are ingested as drugs, food additives or as trace contaminants of food with agricultural chemicals. At least 20 such amines have been given chronically to rats together with sodium nitrite and the incidence of tumors compared with that in rats given the amine or nitrite alone. Until now 13 of these amines have given rise to a significant incidence of tumors under these conditions, indicating that ingestion of these amines when nitrite is present could provide a source of carcinogenic nitrosamines for man. some of the amines were more effective in this respect than others. These were aminopyrine, morpholine, disulfiram, methylbenzylamine and heptamethyleneimine.