Intestinal tumors were induced in male Sprague-Dawley rats following the administration of five weekly doses of 1,2-dimethylhydrazine (DMH) by gavage. At intervals thereafter, groups of rats were given indomethacin (IND) in the drinking water. In three trials, the incidence of rats with tumors was significantly reduced by 40% below the control rats which did not receive IND and there was no significant difference in body weights. Also, in those IND-treated rats which developed tumors, the tumors were generally smaller in numbers and sizes compared to the control rats. A group of DMH-treated rats was treated with crude IND by gavage and the differences between them and untreated control rats were not significant. It is likely that the treatment was directed at the tumors and not at the DMH which induced them.