The effects of indomethacin on the urinary bladder and renal pelvis in rats treated with N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide (FANFT) were studied. Two hundred female Sprague-Dawley rats were divided into four groups. Group 1 received control diet without added chemicals. Group 2 was treated with indomethacin (1 mg/kg per day) in the drinking water throughout the experiment. Groups 3 and 4 received 0.2% FANFT in the diet for seven weeks followed by control diet. In addition to FANFT, Group 4 received indomethacin, 1 mg/kg per day, for the entire experiment. The rats were sacrificed after 92 weeks. There were no urothelial tumors in the control group, one renal pelvic tumor in the indomethacin group, 4 tumors in the FANFT group and 10 urothelial tumors in the FANFT + indomethacin group. The difference between Groups 3 and 4 was statistically significant (P < 0.05). Moderate and severe hyperplasia of the renal pelvic and papillary epithelium was found in 15 of 48 rats in Group 2 (indomethacin only) as compared with 6 of 49 control rats (P < 0.05). Moderate and severe hyperplasia was equally frequent in Groups 3 and 4 (14 and 17 animals in each group, respectively). Twenty-four rats in Group 2 had mammary tumors as compared to 12 animals in Group 1 (P < 0.01). Five of the tumors in Group 2 were adenocarcinomas. There was no difference between the number of mammary tumors in Groups 3 and 4 (36 and 32 animals in each group, respectively). The results suggest that indomethacin enhances FANFT-induced urinary tract carcinogenesis. Indomethacin also seems to exert some tumorigenic activity in the mammary gland.