Both heterocyclic amines and a high fat diet are associated with an increased risk of cancer in many organs. Female A/J mice were fed a diet supplemented with 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and a high fat diet to test for the development of lung tumors. In experiment 1, the mice were divided into 6 groups. Groups 1, 2, 3 and 4 were fed a diet supplemented with MeIQx at a concentration of 600?ppm for 0-12 weeks. A high fat diet containing 20% corn oil was given to Groups 1 and 5 for 0-32 weeks, Group?2 for 12-32 weeks and Group 3 for 0-12 weeks. Group 6 was fed a basal diet without supplements. MeIQx-treated groups (Groups?1, 2, 3 and 4) showed a significant increase in macroscopic and microscopic lung nodules compared with the control (Group 6). Areas of adenomas were increased dependent on the duration of exposure to the high fat diet. In experiment 2, Group 1 mice were fed MeIQx and a high fat diet, Group?2 a MeIQx alone diet, Group 3 a high fat alone diet, and Group?4 a basal diet without supplements. CYP1A2 mRNA in the liver was significantly decreased by a high fat diet (Group?3). The MeIQx alone group (Group 2) showed a tendency towards increased CYP1A2 expression, which was partially reduced in the MeIQx + high fat-treated group (Group 1). In the lungs, CYP1A2 mRNA expression was at an extremely low level, with no intergroup differences. In conclusion, MeIQx exerts tumorigenic potential in the lungs, and a high fat diet increases the size of induced lesions. The expression level of CYP1A2 in relation to MeIQx and a high fat diet may be associated with lung carcinogenesis.