The epithelial lining of the epididymal duct expresses the androgen receptor (Ar) along its entire length and undergoes rapid and profound degeneration when androgenic support is withdrawn. However, experiments involving orchidectomy with systemic testosterone replacement, and testicular efferent duct ligation, have indicated that structural and functional integrity of the initial segment cannot be maintained by circulating androgen alone, leaving the role of androgen in this epididymal zone unclear. We addressed this question in a mouse model with intact testicular output and selective Ar inactivation in the proximal epididymis by creating double-transgenic males carrying a conditional Ar(loxP) allele and expressing Cre recombinase under the promoter of Rnase10, a gene specifically expressed in proximal epididymis. At 20-25 d of life, on the onset of Rnase10 expression, Ar became selectively inactivated in the principal cells of proximal epididymis, resulting in epithelial hypoplasia and hypotrophy. Upon the subsequent onset of spermiation, epididymal obstruction ensued, with the consequent development of spermatic granulomata, back pressure-induced atrophy of the seminiferous epithelium, orchitis, and fibrosis of the testicular parenchyma. Consistent with these findings, the mice were infertile. When the effect of Ar knockout on gene expression in the proximal epididymis was compared with that of efferent duct ligation and orchidectomy, we identified genes specifically regulated by androgen, testicular efferent fluid, and both. Our findings demonstrate that the development and function of the epididymal initial segment is critically dependent on direct androgen regulation. The phenotype of the produced knockout mouse provides a novel model for obstructive azoospermia.