&NA;Seventy‐five male Sprague‐Dawley rats received weekly injections of dimethylhydrazine (DMH) for six months. Indomethacin was given orally to 40 of the rats. At the end of the allotted period, colonic tumors were autotransplanted into the subcapsular space of the left kidney. The autopsy demonstrated colonic tumors in 32 of 34 rats treated with DMH (94.1 percent), but only in 15 of 40 rats (37.5 percent) treated with DMH‐indomethacin. Successful kidney autotransplantation was accomplished in 20 of 32 DMH‐treated rats (63 percent) and in 5 of 15 DMH‐indomethacin‐treated rats (33.3 percent) Thus, the induction of colonic tumors by DMH and successful kidney autotransplants can be substantially abrogated by synchronous treatment with indomethacin.