Indole-3-carbinol (I3C) present in cruciferous vegetables has been found to inhibit chemically induced neoplasms in forestomach, mammary gland, liver, and tongue in rodents. I3C is also known to induce estradiol 2-hydroxylase and reduce estrogenic activity. The current study was conducted to examine the possible inhibiting effect of I3C on spontaneous occurrence of endometrial adenocarcinoma in female Donryu rats. The high incidence of endometrial cancer in this strain of rats might be related to increased estrogen/progesterone ratio with aging. A total of 141 female Donryu rats were divided into four groups. Group 1 was given a basal diet alone throughout the study. Starting at 6 weeks of age, groups 2-4 were respectively given the diets containing 200, 500, and 1000 ppm I3C for the entire experimental period (660 days). At the termination of the experiment, the incidences of preneoplastic lesions and neoplasms in the endometrium were estimated. Also, estradiol 2-hydroxylation activity in the liver was assayed in rats fed I3C at these doses for 3 weeks. The incidences of endometrial adenocarcinoma in rats fed I3C [8 of 32 rats (25%) in group 2, 5 of 32 rats (16%) in group 3, and 5 of 35 rats (14%) in group 4] were respectively smaller than that in group 1 (12 of 32 rats, 38%). The incidence of uterine adenocarcinoma in group 4 was significantly lower than that in group 1 (P < 0.05). Dietary I3C also decreased the frequency of preneoplastic endometrial lesions (31% in groups 2-4 versus 44% in group 1). I3C exposure also inhibited the incidence of mammary fibroadenoma. Biochemical assay for estradiol 2-hydroxylation revealed that feeding of I3C significantly increased estradiol 2-hydroxylation (nmol/mg protein) compared to rats treated without I3C [0.34 +/- 0.04 in group 2, 0.53 +/- 0.13 in group 3, and 0.58 +/- 0.11 in group 4 versus 0.28 +/- 0.02 in group 1; P < 0.02, P < 0.003, and P < 0.001, respectively]. These results suggest that dietary I3C inhibits spontaneous occurrence of endometrial adenocarcinoma as well as preneoplastic lesions in Donryu rats. This chemopreventive effect of I3C may be due to its induction of estradiol 2-hydroxylation.