Aspirin and indomethacin, administered systemically by oral route, were found to delay the development of hamster buccal pouch epidermoid carcinomas induced by thrice weekly topical applications of a 0.5 percent solution of 7,12-dimethylbenz(a)anthracene (DMBA) in mineral oil. Forty male and female Syrian hamsters (Mesocricetus auratus) were divided into four equal groups. In Group 1 animals the left buccal pouch was painted thrice weekly with DMBA. Group 2 animals were painted thrice weekly with DMBA and received 12 mg. aspirin daily by oral route. Group 3 animals were painted thrice weekly with DMBA and received 1 mg. indomethacin daily by oral route. Group 4 animals were maintained as untreated controls. Two animals in each of the four groups were killed with ether at 8, 10, 12, 13, and 14 weeks after the start of the experiment. At the time of sacrifice the buccal pouches were photographed and the average number of tumors and the average size of tumors in each group were noted. The left and right buccal pouches were dissected, fixed in 10 percent formalin, sectioned in paraffin, and stained with hematoxylin and eosin. Autopsies were also performed on each animal. Both left and right buccal pouches and major organs were studied histologically. Both aspirin and indomethacin in the dosages used were found to delay DMBA buccal pouch carcinogenesis. A suggested mechanism of action is the inhibition of prostaglandin synthesis by the role of both aspirin and indomethacin as inhibitors of prostaglandin synthetase. Indomethacin appeared to exert a greater tumor-inhibiting effect than aspirin in the dosages used.