SummaryAdministration of 0.01% unsymmet,rical 1,1-dimethylhydrazine in the drinking water of 5-week-old randombred Swiss mice throughout their lifetime induced tumors of blood vessels, lung, kidney, and liver. The incidences of tumors in these 4 tissues were: 79, 71, 10, and 6%, while in the untreated controls they were 2, 11, 0, and 0%, respectively. The vascular tumors occurred in the liver, muscle, heart, lung, and fat. Macroscopic, light and electron microscopic, and histochemical examinations of the vascular lesions revealed a morphology typical of angiosarcomas. The gross and histologic involvement of the tissues with the tumors is described and illustrated. The ultrastructure of hemorrhagic areas, angiosarcoma with its components, and the neoplastic endothelial cells and their associated structures and membranes are shown in detail. The findings of this study prove unequivocally the carcinogenicity of this important rocket propellent and provide further evidence for a possible relationship between the chemical structure of hydrazine derivatives and tumor development at specific organ site.