Keratinocyte growth factor (KGF/FGF-7) is a stromally derived factor which exerts proliferative and differentiating effects on a variety of epithelial cells. Results of recent studies utilizing in vitro methods such as tissue culture and organ culture have suggested that KGF may act as a paracrine mediator of androgen-induced growth and development of the prostate and seminal vesicle. We undertook the present study to determine the distribution of KGF in relation to the functional regions of the rat prostatic ductal system, and whether KGF expression is influenced by androgen in vivo. Immunohistochemical staining revealed KGF to be present in the stroma throughout the prostate, regardless of the functional region, and staining for KGF remained high through 21 days post-castration. Message for KGF could also be detected by reverse transcriptase-PCR analysis of prostate stromal cells isolated from 4-and 21-day castrated animals, and no gross change in message level was observed following castration. Furthermore, no significant change in either stromal staining or message for KGF was observed in newborn rat prostates 10 days after castration, suggesting a similar regulatory mechanism for KGF in the adult and immature prostate. Epithelial staining for KGF decreased following castration, and greatly increased upon androgen replacement, possibly indicating a change in KGF internalization. These observations suggest that the presence of KGF protein is not related to functional differences in the prostate epithelium, and that expression of KGF in vivo is not greatly influenced by androgen.