The existence of possible threshold dose levels of genotoxic carcinogens for carcinogenesis is of particular interest for human risk assessment. Recently, no observed effect levels (NOELs) for various hepatocarcinogens have been reported. However, reports on threshold levels for lung carcinogenesis have hitherto been lacking. In the present study, we first investigated low dose response lung and colon carcinogenesis with a food-derived genotoxic carcinogen, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) (0, 0.01, 0.1, 1, 10 and 100 ppm in the diet) alone for 32 weeks using female A/J mice. The endpoints were histopathologically diagnosed hyperplasias and adenomas in the lung, and aberrant crypt foci (ACACF) in the colon. The results showed NOELs of 100 and 1 ppm, respectively. We next investigated the effect of additional pre-treatment with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) (2 mg/mouse, single dose, intraperitoneal injection) prior to the low-dose application of MeIQx (0, 0.01, 0.1, 1, 10, 100 and 600 ppm in the diet) for 32 weeks. Lung lesions were significantly increased in the NNK + MeIQx (1 ppm) group, but not in the NNK + MeIQx (≥10 ppm) groups. Since the dose-response curve was not of so-called 'hockey stick type', it was not possible to determine a NOEL for lung tumorigenesis. Significant increase in the mRNA expression of CYP2A5, a major metabolic enzyme for NNK, was also observed in the NNK + MeIQx (1 ppm) group, and a similar pattern was noted for O6-methylguanine DNA methyltransferase (MGMT). By contrast, the formation of colon ACACF showed a dose-dependent increase. The NOEL for the formation of colon ACACF was considered to be 10 ppm MeIQx with NNK. These results suggest that MeIQx may have different threshold dose levels for the induction of lung tumorigenic lesions and ACACF formation in the colon. Pre-treatment with NNK, a potent lung carcinogen, concealed the effects of MeIQx in the lung, but exerted minimal influence in the colon. CYP2A5 and MGMT expression may be of importance, particularly in the lung. The present study provides critical suggestions for the human risk assessment of genotoxic carcinogens.