In most previous studies, the incidence and multiplicity of chemically induced prostate tumors have been used as end points for assessing the efficacy of various chemopreventive agents. In this study, we used prostate intraepithelial neoplasia (PIN) in Noble rats as an intermediate end point to examine the chemopreventive efficacy of two retinoids, 9-cis-retinoic acid (9cRA) and 4-(hydroxyphenyl)retinamide, which in previous studies have shown promising inhibitory effects on various carcinogenesis models. We found that 80-100% of Noble rats treated for 36 weeks with testosterone + 17beta-estradiol developed multiple PIN lesions predominantly in the dorso-lateral prostate, which appears relevant to the place of origin of PIN and carcinoma in the human prostate. 9cRA at 50 or 100 mg/kg diet significantly decreased the multiplicity of PIN, whereas 4-(hydroxyphenyl) retinamide at 392 or 784 mg/kg diet, did not have an inhibitory effect on PIN. Thus, we provide for the first time evidence that the testosterone + 17beta-estradiol-induced PIN in Noble rats could be used as a potential intermediate end point in assessing the efficacy of retinoids and possibly of other agents on prostate carcinogenesis, and that 9cRA alone or in combination with other agents may have clinical promise in preventing the development of prostate cancer in men.