Transgenic (Tg) mice expressing human fibroblast growth factor 8b (FGF8-b) under the probasin promoter (Tg [Pbsn-FGF8] L2-L5Elo; hereafter referred to as FGF8-b-Tg) were shown to produce FGF8-b at high levels in the prostate and epididymis and at lower levels in the testis. The present study examined the effects of FGF8-b expression on the epididymis and testis. In old (age, >6 mo) FGF8-b-Tg mice, epididymides were frequently enlarged, with epithelial and stromal hypercellularity progressing upon aging to epithelial dysplasia and malignant transformation of stroma. In addition, oligospermia, dilatation of the duct, and inflammation were frequently observed in the epididymides. In association with the epididymal changes, some FGF8-b-Tg mice presented a degenerative seminiferous epithelium of the testis. Consistent with this observation, infertile males were found in two FGF8-b-Tg mouse lines. Masson trichrome staining and immunohistochemical analysis of smooth muscle actin, laminin, and androgen receptor revealed that changes in the epididymal stroma closely resembled those previously found in the prostates of the FGF8-b-Tg mice. Genes previously found to be upregulated in the prostate of FGF8-b-Tg mice, such as osteopontin (Spp1) connective tissue growth factor (Ctgf), apolipoprotein D (Apod), and FGF receptor 1c (Fgfr1-c), were also upregulated in the epididymides, suggesting that similar molecular mechanisms were active in both tissues. However, unlike in the prostate, the changes in the epididymal epithelium of the FGF8-b-Tg mice did not progress into invasive carcinoma. The results suggest that prolonged and enhanced FGF signaling induces dramatic changes in the epididymis and testis that lead to infertility in a portion of the FGF8-b-Tg males.