The purpose of the present study was to examine whether Neem leaf (Azadirachta indica) has short-termchemopreventive effects on endpoint preneoplastic lesions involved in rat colon carcinogenesis and might also exertantioxidative activity. Forty- two male F344 rats were randomly divided into 6 experimental groups. Groups 1 to 4were given a subcutaneous injection of azoxymethane (AOM, 20 mg/kg body weight) once a week for 2 weeks.Starting one week before the first injection of AOM, rats in groups 2 to 4 received an aqueous extract of Neem leaf(20, 100, and 250 mg/kg, respectively) by gavage 3 times per week, for 5 weeks. Rats in group 5 also were given theNeem extract by gavage feeding 3 times per week for 5 weeks, while group 6 served as untreated controls. Theexperiment was terminated 5 weeks after the start. Dietary feeding of the Neem extract at all dose levels significantlyinhibited the induction of aberrant crypt foci (ACF) (P<0.0002), when compared to the AOM-treated group (group1). In groups 2 to 4, treatment of rats with the Neem extract also significantly decreased the proliferating cell nuclearantigen (PCNA) labeling indices (P<0.0006) of colon epithelium and ACF. Moreover, the Neem extract also showedantioxidative activity. The finding that dietary Neem has possible chemopreventive effects in the present short-termcolon carcinogenesis bioassay suggests that longer-term exposure may cause suppression of tumor development.