BACKGROUND 2-methylimidazole is used to make many other chemicals for drugs, photography, dyes, rubber, and agriculture. We studied the effects of 2-methylimidazole on male and female rats and mice to identify potential toxic or cancer-related hazards to humans. METHODS We studied the effects of 2-methylimidazole by mixing it in the feed of rats and mice for 2 years. The doses given were 300, 1,000, or 3,000 parts per million (ppm) 2-methylimidazole (equivalent to 0.03%, 0.1%, or 0.3%) for male rats; 1,000, 2,500, or 5,000 ppm for female rats; and 625, 1,250, or 2,500 ppm for male and female mice. There were 50 animals in each exposure group. Control animals received the same feed with no chemical added. Tissues from more than 40 sites were examined for every animal. RESULTS For both male and female rats and mice, the groups receiving the highest amounts of 2-methylimidazole weighed less on average than the control animals. Male and female rats and male mice receiving 2-methylimidazole had higher rates of thyroid gland cancers than did the untreated control animals. The rates of liver tumors were greater in male and female mice receiving 2-methylimidazole and also slightly increased in male and female rats receiving 2-methylimidazole. CONCLUSIONS We concluded that 2-methylimidazole caused increased rates of cancer of the thyroid gland and liver in rats and mice.