There is evidence that estrogens can modulate the activity of prostate epithelial cells. To determine whether estradiol can have a direct influence on rat prostate, this study examined the effects of estradiol-17β (E₂) administered alone or in combination with dihydrotestosterone (DHT) to castrated rats for 3 weeks on prostate binding protein (PBP) C1 mRNA expression and androgen receptor (AR) localization. PBP C1 mRNA levels were measured by semi-quantitative in situ hybridization using a ³⁵S-labeled cDNA probe. In intact animals, strong hybridization signal could be observed in prostate sections after 12 hr of exposure to Kodak X-Omat films. In castrated rats, no PBP C1 mRNA could be detected even with longer exposure times, an effect that was prevented by administration of DHT. E₂ administered alone induced a detectable hybridization signal, and the concomitant administration of E₂ and DHT induced an increase in PBP C1 mRNA that significantly exceeded that obtained in animals that received only DHT. In prostate epithelial cells of intact animals, AR immunostaining was restricted to the nucleus. In castrated animals the alveoli were decreased in size and the epithelial cells were atrophied. AR staining was weak and was detected in both cytoplasm and nucleus. DHT administration completely obviated the effect of castration on epithelial cell histology and on AR immunostaining distribution and intensity. Interestingly, E₂ administration alone induced moderate hypertrophy of epithelial cells compared to the histological appearance of cells in untreated castrated rats. Moreover, in E₂-treated animals the nuclear staining was much stronger than that detected in untreated castrated rats, whereas the cytoplasmic staining was not modified by the treatment. In animals that received both DHT and E₂, the staining was similar to that seen in DHT-treated rats. These results suggest that E₂ can influence the activity of rat prostate epithelial cells by mechanisms that remain to be fully clarified.