Four different experiments were performed in order to examine the modifying effects of chlorogenic acid (CA), reserpine, polyprenoic acid (E-5166), and coffee on chemical carcinogenesis in rats or hamsters. Experiment 1: The numbers of hyperplastic liver cell foci and the incidence of colon tumors in male and female Syrian golden hamsters given a single intravenous injection of methylazoxymethanol (MAM) acetate and then fed the diet containing 0.025% CA for 24 wk were significantly lower than those of hamsters given MAM acetate alone. Experiment 2: The incidence of altered hepatocellular foci in female ACI/N rats given N-2-fluorenylacetamide (FAA, 0.02% in diet) for 10 wk and reserpine (weekly subcutaneous injections, 1 microgram/g body weight) during or after (17 wk) FAA exposure was significantly lower than that of rats given FAA alone. Experiment 3: The number of hepatocellular foci in male ACI/N rats given 0.02% FAA diet for 13 wk and E-5166 by gavage (40 mg/kg body weight, 3 times/wk) for 16 wk after the end of FAA exposure was significantly smaller than that in rats given FAA diet alone. Experiment 4: Incidences of liver tumors and hepatocellular foci of rats given concurrent dietary administration of aminopyrine (0.01%) and sodium nitrite (0.1%) and coffee solution as a drinking water for 630 da were significantly lower than those of rats given aminopyrine and sodium nitrite. Thus, the tested compounds had inhibitory effects on chemical carcinogenesis in liver or colon.