We have previously reported that dietary neem flowers (Azadirachta indica A. Juss var. siamensis Valeton) caused a marked increase in glutathione S-transferase (GST) activity in the liver, while resulting in a significant reduction in the activities of some hepatic P450-dependent monooxygenases. These results strongly indicate that neem flowers may have chemopreventive potential. In the present study, we examined the inhibitory effects of neem flowers on 9,10-dimethyl-1,2-benzanthracene (DMBA)-induced mammary gland carcinogenesis in female Sprague Dawley rats and on aflatoxin B 1 (AFB1)-induced hepatocarcinogenesis in male Wistar rats. Young animals were fed with AIN-76 purified diets containing either 10-12.5% ground freeze-dried neem flowers for 1 week prior to, during, and for 1 week after the administration of each carcinogen. Interestingly, it was found that neem flowers resulted in a marked reduction of the incidence of mammary gland (about 35.2%) and liver tumors (61.7% and 80.1% for benign and malignant tumors, respectively). Furthermore, the multiplicity of tumors per rats was also lower in the neem flower groups, i.e. those for mammary gland tumors and benign and malignant liver tumors were reduced to 44.0%, 87.9% and 88.9%, respectively. These results clearly demonstrated that neem flowers contain some chemopreventive agents capable of inhibiting AFB1 and DMBA induced liver and mammary gland carcinogenesis in rats.