In a long-term experiment the induction of tumors in rats after oral application of aminopyrine and nitrite by gastric intubation was investigated. The animals treated with aminopyrine and nitrite (test-group) had an incidence of different types of liver tumors of 96% in females and 97% in males. The percentage of liver tumors was increased significantly in the test-group in comparison to all control-groups. Besides the tumors of the liver other types of tumors were stated to a minor degree, but there was no difference between the test-group and the control-groups.