The incidence of clinically manifested prostate cancer is higher in the United States and Europe than in Asian countries. One of the major differences in lifestyle between these populations is the diet, with Asians consuming a greater amount of soy. Soy and genistein, the predominant isoflavone found in soy, inhibit prostate tumor development in animal models. The purpose of this study was to investigate the effect of dietary genistein on sex steroid receptor expression in the dorsolateral prostate, on circulating androgens, and the potential for toxicity in the male rat reproductive tract. Male Sprague-Dawley rats were fed 25 and 250 mg genistein/kg diet from conception until day 70 postpartum, or 250 and 1000 mg genistein/kg diet from day 56 to 70 postpartum. Exposure to genistein in the diet, starting at conception, resulted in down-regulated androgen receptor (AR), and estrogen receptors (ER)-α and -β mRNA expression in the dorsolateral prostate in a dose-dependent manner. Also, genistein fed to adult rats for 2 weeks reduced mRNA expression of AR, ER-α and ER-β in the dorsolateral prostate. ER-α protein levels were significantly reduced in animals fed 1000 mg genistein/kg diet compared to control animals. There were no significant alterations to male reproductive tract histomorphology or weights. We conclude that dietary genistein down-regulated expression of the AR and ER-α and -β in the rat prostate at concentrations comparable to those found in humans on a soy diet. Down-regulated sex steroid receptor expression may be responsible for the lower incidence of prostate cancer in populations on a diet containing high levels of phytoestrogens. Copyright © 2002 Elsevier Science Ireland Ltd.