Chemoprevention is a practical approach to control colorectal cancer, which is one of the major causes of cancer mortality in the United States. Based on our recent silibinin efficacy studies in human colorectal cancer cells, we investigated the effects of its dietary feeding on azoxymethane (AOM)-induced aberrant crypt foci (ACF) formation and associated biomarkers in male Fisher 344 rats. Five-week-old male Fisher 344 rats were fed control or silibinin-supplemented (0.033%, 0.1%, 0.33%, or 1%, w/w) diet. After 2 weeks, AOM was injected once a week for 2 weeks while silibinin treatments were continued. In another protocol, identical silibinin treatments were done but started 2 weeks post-AOM initiation. All rats were sacrificed at 16 weeks of age, and colon samples were evaluated for ACF, followed by proliferation, apoptosis, and inducible nitric oxide synthase and cyclooxygenase-2, by immunohistochemistry and/or immunoblotting. Silibinin significantly (P < 0.001) reduced dose-dependently the number and multiplicity of AOM-induced ACF formation. Silibinin feeding in pre- and post-AOM initiation decreased mean number of ACF by 39% to 65% and in post-AOM initiation by 29% to 55%. Silibinin dose-dependently decreased AOM-induced colonic cell proliferation, evidenced by proliferative cell nuclear antigen and cyclin D1 immunohistochemical staining, and induced apoptosis in these colon tissues, evidenced by terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling staining and cleaved poly(ADP-ribose) polymerase. Furthermore, silibinin significantly decreased AOM-induced inducible nitric oxide synthase- and cyclooxygenase-2-positive cells in colon tissues. The present findings show possible beneficial activity of silibinin at least in early stage of colon tumorigenesis, suggesting that silibinin might be an effective natural agent for colorectal cancer chemoprevention.