Syrian golden hamsters were treated with N delta-(N-methyl-N-nitrosocarbamoyl)-L-ornithine (MNCO), a nitrosourea amino acid, which has induced a high incidence of breast, kidney and skin neoplasms and a low incidence of pancreatic carcinoma in rats. MNCO induced a few breast and skin carcinomas, and a high incidence of foci of atypical acinar cells and of focal ductular abnormalities in the exocrine pancreas. The latter were similar to lesions observed in rats treated with MNCO. MNCO was less toxic and less effective as a carcinogen in hamster than in the rat.