tThe aim of this study is to develop a novel experimental model of the subcutaneous transplantation offetal urogenital sinus (UGS) into normal and castrated adult male rats for the pathophysiological inves-tigation of the normal and developing prostate. Fetal UGS obtained from 20-day-old male rat embryoswas subcutaneously transplanted into 7-week-old normal and castrated male rats. We observed thegrowth pattern, histopathological characteristics and immunohistochemical localization of cytokeratin5 (CK 5), cytokeratin 8 (CK 8) and androgen receptor (AR) in the transplanted tissues. Almost all of thetransplanted UGS organs gradually increased in weight over time in the non-castrated recipient animals,and the histopathological observations and immunohistochemical analysis of CK 5 and CK 8 revealedthat the morphological changes in the tissues were in accordance with the features of normal prostatedevelopment. The histological characteristics included glandular epithelial dominant and stromal domi-nant area, with an increase in the glandular epithelial dominant areas over time and resemblance amonga portion of the transplanted tissues within a certain period during the developmental course to thehistopathology of human benign prostatic hyperplasia (BPH). The effects of androgens and resemblancein the immunohistochemical localization pattern changes in AR to that observed in the normal differ-entiating rat prostate were also noted. We conclude that the subcutaneous space provides an adequate