Lifetime administration of 0.03125% 1,1-di-n-butylhydrazine in drinking water to Swiss mice, from 6 weeks of age, induced tumors of the lungs, forestomach and liver. The tumor incidences in these tissues in untreated controls were 25, 2 and 0.5%, whereas in the treated groups the corresponding tumor incidences increased to 68, 39 and 5%, respectively. Histopathologically these lesions were classified as adenomas and adenocarcinomas of the lungs, squamous cell papillomas and carcinomas of the forestomach, and benign hepatomas and liver cell carcinomas. The work is part of a structure activity inquiry and its specific aim is to disclose whether the dialkyl derivatives of hydrazine are more active carcinogens than the monoalkyl analogues.