As a preliminary study to elucidate whether Plantago ovata forsk (PO) exhibits inhibiting effects on mammary carcinogenesis under a condition of hypercholesterolemia, 50 female Sprague-Dawley rats were first given a single intragastric dose of 200 mg/kg body weight of 7, 12-dimethylbenz[a]anthracene (DMBA). Ten of 50 rats died of acute toxicity of DMBA within one week. From one week later, the survived animals were divided into 4 groups: 13 rats in group 1 and 7 in group 2 received high cholesterol diets (HC) with and without 5% PO supplementation for 26 weeks, respectively. Eleven rats in group 3 and 9 in group 4 received basal diet (BD) with and without 5% PO supplementation for the same period, respectively. One to three rats in groups 1 to 4 died of mammary tumors, lymphomas and/or adrenal impairment attributable to DMBA treatment during the treatment period. Group 1 showed a tendency to decrease in the number of palpable mammary masses as compared with group 2, whereas group 3 showed a tendency to higher values compared with group 4. At the termination of the study, the serum levels of total cholesterol in group 1 were significantly lower than those in group 2 and the number of mammary masses was significantly decreased. Histopathologically, this decrease was mainly due to the decreased incidences of mammary adenocarcinomas, while no significant difference in the multiplicity of mammary tumor was observed between BD+PO and BD alone groups. The results of the present study suggest a possibility that PO exerts inhibiting effects on mammary carcinogenesis by decreasing circulating cholesterol levels in a rat two-stage mammary carcinogenesis model.