N-Methyl-N-formylhydrazine (MFH) was administered in drinking water as a 0.0078% solution to randomly bred Syrian golden hamsters for life beginning at 6 weeks of age. The treatment gave rise to benign and malignant liver cell tumors, malignant histiocytomas and tumors of the gall bladder and bile ducts. The tumor incidence in these four treated tissues was 43, 34, 11,8%, while in untreated controls it was 0, 0, 0, 0%, respectively. Histopathologically, tumors were classified as benign hepatomas, liver cell carcinomas, malignant histiocytomas, adenomas and adenocarcinomas of the gall bladder, cholangiomas, and cholangiocarcinomas.