The dose-dependence of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) carcinogenicity when given in the diet continuously throughout the experiment (56 weeks) at doses of 100, 200 and 400 ppm, was examined in F344 male rats. Tumors developed dose-dependently in the liver, Zymbal glands and skin in the experimental groups, but were lacking in the control group. Benign tumors such as adenomas and papillomas were found in rats given 100 ppm MeIQx, whereas carcinomas were induced by 200 and 400 ppm MeIQx treatments. The incidences of hepatocellular carcinomas in the 100, 200 and 400 ppm MeIQx groups were 0, 45 and 94%, and of Zymbal gland squamous cell carcinomas were 0, 10 and 56%, respectively. Thus, the response in terms of carcinoma development in these organs in relation to dose of MeIQx seemed to be non-linear.