To analyze the effect of chemical carcinogens on urothelial and intestinal mucosa on 214 female Wistar rats an ureterosigmoidostomy was performed. After 10-14 days 123 surviving rats were randomly divided into 4 groups: Group I-III received 0.05 per cent N-Butyl-N-(4-hydroxybutyl)-nitrosamine (BBN) in drinking water over a period of 15 weeks. Group IV received normal drinking water over the same period. The defunctionalized bladders were instilled each second day: Group I: physiologic saline solution, Group II: urine of normal rats, Group III and IV: urine of BBN-treated rats. 30 rats without diversion but BBN treatment served as a control. The evaluation of the histological data gave the following results: In the control group urothelial tumors were found in the bladder exclusively. Dependent on the grade of obstruction in the BBN-treated groups, with diversion urothelial tumors in the renal pelvis and ureter could be documented, whereas in the bladder no tumor growth could be shown. In the intestinal mucosa of BBN-treated animals a high incidence of adenocarcinoma was found. The chemical tumor induction by BBN is related to the urine and takes place by direct contact to the mucosa when the metabolites excreted by the kidneys are activated in the urine. BBN or other urogenic carcinogens seem to have no urothelial specificity. The incidence of bowel carcinoma after diversion must lead to intensive long-term follow up.