The effect of consumption of caraway seeds and dried leaves of watercress on 7,12-dimethylbenz(alpha)anthracene (DMBA)-induced mammary gland tumorigenesis in female Sprague-Dawley rats was determined. At 6 weeks of age, animals were fed a basal (control) diet and experimental diets containing caraway or watercress (20%). Animals were maintained on their diets till termination of the experiment (25 weeks after DMBA). At 8 weeks of age, all rats were given DMBA 10 mg, one dose, p.o. in oily formulation). Neither caraway nor watercress affected the body weight or the rate of growth of animals. By week 25 after DMBA, 77% of the control rats developed mammary tumors, with a mean of 2.27 tumors per rat. Caraway decreased significantly (P<0.05) the percentage of rats with tumors (42.8% protection) and the mean number of tumors per rat (50.6% protection) and increased significantly (P<0.05) the mean latency period of tumor appearance. Watercress, though decreased the percentage of rats with tumors (28.5% protection) and the mean number of tumors per rat (31.7% protection), this decrease was significant (P < 0.05) during some week intervals only. The increase in the mean latency period of tumor appearance by watercress was not significant. The results of this study suggest that caraway and watercress possess chemopreventive effects against DMBA-induced mammary gland tumorigenesis in rats.