AbstractMany furan‐containing natural products that induce increased activity of the glutathione S‐transferase (GST) enzyme system have been found to inhibit tumorigenesis in laboratory animals. 2‐n‐Heptylfuran (HF) and 2‐n‐butylthiophene (BT), a sulfur analogue of furan, are two of the many furans and thiophenes formed during the roasting process of meat. BT and HF, when administered by gavage at doses that ranged from 11 to 90μmol, induced increased GST activity in various tissues of A/J mice. At 90μmol/dose, BT induced increased GST in the liver, small bowel mucosa, and lung. No increase in enzyme activity was found in the forestomach. HF was an enzyme inducer in the liver, small bowel mucosa, and forestomach but was inactive in the lung. The acid‐soluble sulfhydryl level, a good measure of glutathione contents in tissues, was examined in tissue homogenates from mice treated with BT and HF. BT induced significant increase of GSH in the liver and lung at the higher doses. No change was observed in either the small bowel mucosa or the forestomach. A 50‐μmol dose of HF was found to increase GSH level in all four tissues studied. The inhibition of lung and forestomach tumorigenesis was carried out with A/J mice using benzo[a]pyrene as the carcinogen. BT treatment resulted in a reduction of tumor multiplicity in the lung and forestomach. The tumor incidence in the forestomach was reduced significantly. The potency of HF as inhibitor of carcinogenesis was similar to that of BT in the forestomach of mice. In the lung, HF was found to be ineffective as an inhibitor of pulmonary adenoma formation. These results suggest that furan‐and thiophene‐containing compounds may be effective inhibitors of chemical carcinogenesis.