N-Methyl-N-formylhydrazine was administered in drinking water as a 0.0039% solution to randomly bred Swiss albino mice for life starting from 6 weeks of age. The compound induced tumors of lungs, livers, blood vessels, gall bladder and bile ducts. The tumor incidences in these five tissues were 77, 46, 21, 10 and 7% while in the untreated controls they were 18, 1, 6, 0 and 0% respectively. Histopathologically, the tumors were classified as adenomas and adenocarcinomas of lungs, benign hepatomas and liver cell carcinomas, angiomas and angiosarcomas of blood vessels, adenomas and adenocarcinomas of gall bladder and cholangiomas. The macroscopic and light microscopic involvement of the tissues with the tumors are described and some of them are illustrated. N-Methyl-N-formylhydrazine is an ingredient of the edible mushroom, the false morel Gyromitra esculenta. The findings are discussed from the viewpoint of a potential human health hazard.