Eighty young adult male and female golden hamsters (Mesocricetus auratus) were divided into four equal groups. The left buccal pouches of the animals in group 1 were painted three times weekly for 24 weeks with 0.1% solution of 7,12-dimethylbenz[a]anthracene (DMBA) in heavy mineral oil. The left buccal pouches of the animals in group 2 were similarly painted with DMBA for 24 weeks, but the animals also received 10 mg ibuprofen (Motrin) administered systemically by the oral route twice weekly. The animals in group 3 received only ibuprofen, and those in group 4 served as untreated controls. After 24 weeks the animals were killed in groups of 16 (two males and two females from each of the four experimental groups) per week and their left buccal pouches studied for the presence of tumors. Sizes of tumors were also recorded. Ibuprofen was found to prevent tumor formation at 25-27 weeks. Some tumors developed in the animals receiving DMBA and ibuprofen by the 28-29 week, but they were fewer in number and smaller than those of the animals receiving only DMBA and not ibuprofen.