A single subcutaneous injection of N-nitrosobis(2-oxobutyl)amine (BOB) and N-nitroso(2-oxobutyl)(2-oxopropyl)amine (OBOB) induced a high incidence of pancreatic ductular neoplasms in Syrian hamsters. Both compounds showed a cytotoxic effect on pancreatic islet cells in toxic doses. Since both compounds, like N-nitroso(2-hydroxypropyl)(2-oxopropyl)amine (HPOP), can form cyclic structures resembling the hexose sugars and the glucose-moiety of the streptozotocin, it can be assumed that the ability of these 2 carcinogens to cyclize is important in their affinity for the pancreas. However, OBOP had a greater pancreatic carcinogenic effect than BOB, the primary target tissue of which was the liver. Hence factors other than cyclization, such as the presence of the 2-oxo group in the aliphatic chains, also appear to be important for the pancreatic carcinogenicity of this class of nitrosamines.