Heterocyclic amines present in cooked foods are known to produce colon tumors in F344 rats at a high incidence, indicating the possibility of involvement of ras gene activation in colon carcinogenesis in rats as in humans. We examined mutations at codons 12, 13, and 61 of the Ki-ras, Ha-ras, and N-ras genes by polymerase chain reaction--direct sequencing in seven colon tumors in F344 rats induced by 2-amino-6-methyldipyrido-[1,2-a:3',2'-d]imidazole (Glu-P-1), 11 induced by 2-amino-3-methylimidazo[4,5-f]quinoline, and nine induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. A Ki-ras gene mutation (G-->T at the second position in codon 12) was found in one Glu-P-1-induced colon adenocarcinoma. None of the other 26 tumors had mutations in any of these three ras family genes. These results indicate that in rats, colon carcinogenesis induced by heterocyclic amines may be induced by alterations of other oncogenes or tumor suppressor genes. We think this experimental system using carcinogens to which humans are exposed is a good model for studying alterations of other genes in human colon tumors in which no Ki-ras alterations are observed.