Carcinogenicity of 2-amino-3,8-dimethylimidazo[4,5-f]-quinoxaline (MeIQx), which is a potent mutagen first isolated from fried beef and widely present in various cooked foods, was tested in both sexes of F344 rats. Rats were continuously given a diet containing 0.04% MeIQx or basal diet and the experiment was finished on day 429. In experimental animals, the incidence of liver, Zymbal gland, clitoral gland and skin tumors was significantly higher than in control animals. The incidence of liver tumors was 100% in males and 53% in females; most liver tumors of males were hepatocellular carcinomas and all liver tumors of females were neoplastic nodules. The incidence of Zymbal gland tumors was 75% in males and 53% in females. Clitoral gland tumors were induced in 63% and skin tumors were observed in 35% of males and 5% of females. Most of these three types of tumors were diagnosed as squamous cell carcinoma. In the control rats, liver, Zymbal gland, clitoral gland and skin tumors were not observed in either sex.