Modifying effects of chlorogenic acid (CA) on carcinogen-induced large bowel carcinogenesis was examined in rats. A total of 150 male F344 rats, 4 weeks old, were divided into 5 groups. At 6 weeks of age, groups 1-3 were given subcutaneous injections of AOM (15 mg/kg body weight) once a week for three weeks. Group 2 was given the diet mixed with CA at the dose of 250 ppm during the initiation phase (5 weeks), and group 3 was exposed to the same diet during the post-initiation phase (32 weeks). Group 4 received the diet with CA throughout the experiment. Group 5 was maintained on the basal diet alone and served as a control. At the termination of the experiment (36 weeks after the start), the incidence of colon tumors in group 2 and 3 demonstrated a tendency for decrease as compared with group 1 although this did not attain significance At this time, the multiplicity of colon tumors of group 2 was significantly smaller than in group 1. In this study, the anti-proliferating cell nuclear antigen (PCNA) indices for non-neoplastic cells of the colon mucosae in groups 2 and 3 were also smaller than in group 1. The data suggest that CA has chemopreventive potential against colon carcinogenesis in rats like that showen in a hamster model with use of methylazoxymethanol acetate.